Several accessibility tests were failing incorrectly because axe-core
compared the component's foreground text color against the #ececec color
used for the body of the sidebar app, whereas the components are in fact
rendered on a white background.

Change a11y tests to assume a white background by default, as that is
the case for _most_ elements in the UI. Individual a11y tests can customize
this as appropriate.

Add accessible label to annotation body markdown editor

Extract repeated references to editor text string into a variable to
make it more obvious how different statements in the test are connected.

Add accessible label to search input field

Bump postcss from 7.0.26 to 7.0.27

Bumps [babel-plugin-mockable-imports](https://github.com/robertknight/babel-plugin-mockable-imports) from 1.7.0 to 1.7.1.
- [Release notes](https://github.com/robertknight/babel-plugin-mockable-imports/releases)
- [Changelog](https://github.com/robertknight/babel-plugin-mockable-imports/blob/master/CHANGELOG.md)
- [Commits](https://github.com/robertknight/babel-plugin-mockable-imports/compare/v1.7.0...v1.7.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.26 to 7.0.27.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.26...7.0.27)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump preact from 10.3.1 to 10.3.2

Bump axe-core from 3.4.2 to 3.5.1

Bump npm-packlist from 2.0.3 to 2.1.0

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.614.0 to 2.619.0.
- [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/v2.614.0...v2.619.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump request from 2.88.0 to 2.88.2

The a11y test failures were fixed in 2c96ef64.

Bumps [request](https://github.com/request/request) from 2.88.0 to 2.88.2.
- [Release notes](https://github.com/request/request/releases)
- [Changelog](https://github.com/request/request/blob/master/CHANGELOG.md)
- [Commits](https://github.com/request/request/commits)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [axe-core](https://github.com/dequelabs/axe-core) from 3.4.2 to 3.5.1.
- [Release notes](https://github.com/dequelabs/axe-core/releases)
- [Changelog](https://github.com/dequelabs/axe-core/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/dequelabs/axe-core/compare/v3.4.2...v3.5.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [preact](https://github.com/preactjs/preact) from 10.3.1 to 10.3.2.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.3.1...10.3.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [npm-packlist](https://github.com/npm/npm-packlist) from 2.0.3 to 2.1.0.
- [Release notes](https://github.com/npm/npm-packlist/releases)
- [Commits](https://github.com/npm/npm-packlist/compare/v2.0.3...v2.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

In my testing with screen readers the meaning was reasonably clear from
the fact that you had to click on a button labeled "Search annotations"
to focus the field and there is a "Search..." placeholder. Nevertheless,
this keeps automated checking tools happy.

A better solution would probably be to re-design the search UI so that there is
enough space for a visible label and information about the search
syntax, as well as enough space for the search field on smaller screens.

Visually I think the purpose of this field is obvious without an explicit label
but it may not be clear to assistive tech users.

Fix a11y for LoggedOutMessage

Enable debugging info for JSX elements in dev builds and tests

Add reply capability to service, `AnnotationOmega`

Refactor `SidebarContentError` for a11y, usability

Add `flex-shrink` rule to login links

Force 4.5:1 contrast on css
Change <a> link to <Button> —since it acts like a button

Refactor `SidebarContentError` to be simpler and provide a clearer
UX for the four possible states of direct-linked errors (states are based
on error type—annotation or group—and whether the user is logged in or
not).

Reuse sidebar-panel design pattern overall.

Provide an option for the user to “clear the error”, i.e., show all
annotations instead of “filtering” by the direct-linked annotation or
group. Provide a button for the user to log in if they are not currently
logged in.

Update `SearchStatusBar` to remove unneeded references to direct-linked
store information, as it will now not render if there is a direct-linked
error.

Add a `flex-shrink` rule to the login links in the `TopBar` component
to assure that they don’t get all squished up when a search is active.

Replace link with `Button` in `AnnotationHeader` (A11y)

Fixes a11y-linting issue.

Part of https://github.com/hypothesis/client/issues/1727

Fix AutocompleteList aria-selecterd prop

Fix a11y lint on SelectionTabs component

Fix a11y for Excerpt

Remove jQuery dependency from highlighter and add test for multiple text nodes

AnnotationOmega: Add very basic expand/collapse for reply threads

As part of an ongoing project to remove the annotator's jQuery
dependency, rewrite the highlighter to avoid it.

This change is also preparation for some upcoming changes to make
highlights more friendly for screen reader users by reducing the
number of `<hypothesis-highlight>` elements created for a run of text in
various situations. Since each `<hypothesis-highlight>` announces its
presence to the screen reader, this gets "noisy" if there are many such
elements.

Convert ‘Log in’ and ‘Sign up’ links to `Button` components

Part of #1727