Convert this service to a class and expand the documentation slightly.
This is a straightforward conversion with no API or internal changes.

Part of https://github.com/hypothesis/client/issues/3298

The current error handling is not that useful in practice because it
just results in a console error. With a test in place we can more easily
change/refactor this in future though.

 - Document parameters and return types
 - Add a link to the documentation for the available options. We
   currently only ever use `minTimeout`
 - Add a note about why the code calls `operation.retry()` even if the
   operation was successful

 - Convert closures into methods

 - Remove the `profileFetchRetryOpts` field that was used only to shorten the
   delay between retries in tests, in favor of mocking
   `retryPromiseOperation` in tests to remove the delay between retries.

 - Change `SessionService` instance creation in tests to allow
   individual tests to run custom setup logic before the service is
   constructed. This was needed due to allow the `serviceConfig` mock to
   take effect when the `SessionService` constructor runs

 - Remove unnecessary custom Sinon sandbox in tests

This is a straightforward conversion, except for the removal of an
unused `state` property on the instance.

Part of https://github.com/hypothesis/client/issues/3298

First scenario tests the injection of the client on child iframe. The
child iframe is made annotatable, by having an
`enable-annotation="true"` attribute and sharing the same origin. The
injection of the annotator happens after the the sidebar is created,
thus testing if the child iframe is able to discover the sidebar iframe
and communicate with it via postMessages.

In the second scenario, the host page contains two children iframes,
each with its own client. Every annotator/sidebar combo on each frame
should work independently from each other and shouldn't pass any message
across frames.

The name was vestigial. Rename it to `_bucketsContainer` to be more
meaningful and indicate that it is private. Several other private fields
have also been prefixed accordingly.

When a PDF page transitions from a rendered to non-rendered state the
text layer element is removed and existing highlights in that layer
briefly become disconnected from the document. As a result they have an
empty/zero client rect as returned by `element.getBoundingClientRect()`.
This caused the bucket-building logic to think that the anchor
had moved to the top of the screen as the `top` and `bottom` coordinates
of the anchor box became zero. This could lead to anchors which should
be in the "below screen" bucket briefly jumping to the "above screen"
bucket. Once re-anchoring completes and the highlight is anchored into a
placeholder element for the non-rendered page, its anchor box becomes
non-empty again and the anchor "jumps back" to the "below screen" box.

This commit adds logic to ignore any highlights which have zero height,
including highlight elements that are disconnected from the document.

Co-authored-by: Robert Knight <robertknight@gmail.com>

Delegator does no longer exits.

The new comment explains the source of the events and its final
destination.

A few lines have been removed as a result of the code simplification.

If the user hovered an annotation card before anchoring in the document
completed, or if an annotation was re-anchored in a PDF after the card
was hovered, then the highlight in the document was incorrectly shown in
an un-focused rather than focused state.

Fix the issue by keeping track of the current set of focused annotations
in the Guest and setting the focus state correctly for new highlights
added by `anchor`.

Part of https://github.com/hypothesis/client/issues/3269

 - Revise the documentation for `createPlaceholder` to better explain
   what a placeholder is and the 1:N relationship between placeholders
   and highlights for a given page. Also clarify what the "approximate
   location" means

 - Rename a variable `isPlaceholder` => `inPlaceholder` as this is more
   accurate

Fix incorrect result if the passed node has no parent.

Several different parts of the code need to know about placeholder
elements used to anchor annotations that refer to not-yet-rendered pages
of a document in a PDF. Centralize the logic for creating placeholders
and testing whether a node is inside a placeholder in one module.

A trivial changes it that the "Loading annotations" text now ends with
an ASCII "..." instead of unicode ellipsis "…". This is easier to type
since the text is not intended to be seen by the user.

Move the logic for scrolling the content to the highlights associated
with an anchor from `Guest` to the content-type specific integration
(`HTMLIntegration`, `PDFIntegration`). The implementations of this logic
in `HTMLIntegration` and `PDFIntegration` are currently the same as
before, but in future this will allow adapting the scrolling logic to
the content type.

In the case of PDFs the logic will need to allow for the fact that an
annotation may be re-anchored to a different element as the document is
scrolled due to the page containing the annotation changing between an
un-rendered and rendered state.

 - Add `scrollToAnchor` method to `Integration` interface and implement
   it for `HTMLIntegration`, `PDFIntegration`
 - Replace `scrollIntoView` call in guest with a call to the active
   integration's `scrollToAnchor` method

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.13.2 to 0.13.3.
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/master/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.13.2...v0.13.3)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [gulp-replace](https://github.com/lazd/gulp-replace) from 1.0.0 to 1.1.1.
- [Release notes](https://github.com/lazd/gulp-replace/releases)
- [Commits](https://github.com/lazd/gulp-replace/commits)
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Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.13.15 to 7.13.16.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.13.16/packages/babel-core)
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Bumps [postcss](https://github.com/postcss/postcss) from 8.2.10 to 8.2.12.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/8.2.10...8.2.12)
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Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.888.0 to 2.892.0.
- [Release notes](https://github.com/aws/aws-sdk-js/releases)
- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.888.0...v2.892.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 18.5.2 to 18.5.3.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v18.5.2...v18.5.3)
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Bumps [core-js](https://github.com/zloirock/core-js/tree/HEAD/packages/core-js) from 3.10.1 to 3.11.0.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/commits/v3.11.0/packages/core-js)
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Bumps [eslint](https://github.com/eslint/eslint) from 7.24.0 to 7.25.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v7.24.0...v7.25.0)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [terser](https://github.com/terser/terser) from 5.6.1 to 5.7.0.
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/terser/terser/commits)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [sass](https://github.com/sass/dart-sass) from 1.32.10 to 1.32.11.
- [Release notes](https://github.com/sass/dart-sass/releases)
- [Changelog](https://github.com/sass/dart-sass/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sass/dart-sass/compare/1.32.10...1.32.11)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 8.0.0 to 9.0.0.
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- [Changelog](https://github.com/puppeteer/puppeteer/blob/main/CHANGELOG.md)
- [Commits](https://github.com/puppeteer/puppeteer/compare/v8.0.0...v9.0.0)
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Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.2.5 to 6.3.1.
- [Release notes](https://github.com/getsentry/sentry-javascript/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/sentry-javascript/compare/6.2.5...6.3.1)
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [redux](https://github.com/reduxjs/redux) from 4.0.5 to 4.1.0.
- [Release notes](https://github.com/reduxjs/redux/releases)
- [Changelog](https://github.com/reduxjs/redux/blob/master/CHANGELOG.md)
- [Commits](https://github.com/reduxjs/redux/compare/v4.0.5...v4.1.0)
Signed-off-by: dependabot[bot] <support@github.com>

Part of https://github.com/hypothesis/client/issues/3298

Checking `typeof <var>` is only needed when the variable may not exist
(eg. some browser global).