In PDFs, highlights are drawn in an otherwise-transparent text layer _on
top_ of a canvas which displays the text that the user sees. Therefore
highlights reduce the contrast of the text below and more than two
levels starts to make the text very difficult to read.

The ideal solution would be to draw the highlights below the text, but
that's not trivial.

As a simple step to improve the situation, make nested highlights that
are 3+ levels deep transparent. This limits the amount of contrast
reduction that overlapping highlights can cause.

In web pages, the highlights are drawn _behind_ the text, so contrast is
not reduced as much, but the same change is applied there for a
consistent experience and also because it is easier to implement.

Partially addresses:

 - https://github.com/hypothesis/client/issues/1995
 - https://github.com/hypothesis/client/issues/408

Fix Hammer.JS error on web pages that use Require.JS

Add button to adder toolbar to show annotations for selection

Remove `onReplyCountClick` prop and use store method instead

Add a button to the adder toolbar to show the existing annotations that
relate to the selected text, along with a keyboard shortcut (the "s"
key) to trigger it. This provides a way to filter annotations in the
sidebar to those that correspond to a particular highlight or region of
text using only the keyboard.

Instead of an icon the "Show" toolbar button displays a badge whose
label is the number of annotations associated with the selection, as
that is more informative.

In implementing this, the CSS for the adder toolbar was refactored to
make the effects of several rules easier to follow and improve readability of
toolbar button labels:

 - Use of `!important` was removed. This used to be for the purpose of
   overriding any page styles which affected the adder. That issue no
   longer applies in most browsers due to the use of Shadow DOM to
   isolate the adder from the page's styles.

   This may cause a regression on some pages in older browsers which don't
   support Shadow DOM, if those pages' have generic CSS rules which
   style generic elements.

 - To make the label easier to read, the font size was slightly
   increased

Fix an error related to Hammer.JS on client startup on pages that define
a global variable called `define`, which is part of Require.JS. Hammer's UMD
bundle does not set `module.exports` in this case, so modules that import it
get an empty object back.

Fix the issue by making sure that any reference to `define` inside
bundles evaluates to undefined (assuming there is no local variable
within the module with that name).

This fixes an issue where clicking "Annotate" failed to open the sidebar
on BBC News articles (eg. https://www.bbc.co.uk/news/world-52144390).

Let `AnnotationHeader` call `store.setCollapsed` directly instead of
passing a callback down through several layers of components

Fix annotation and thread vertical rhythm and layout

Adjust some HTML semantic structural elements

Use SVG icons for adder toolbar

Replace `module.exports` with ES exports

Make the icons for Annotate and Highlight in the adder toolbar
consistent with the sidebar by replacing the icon font icons with the
`SvgIcon` component.

Adjust the vertical margin between the icon and the label slightly as
the new Highlight icon and label look too close otherwise.

Move SvgIcon component to `src/shared/` in preparation for re-use in the
"annotator" part of the application.

Change `cross-origin-rpc.js` to use ES exports instead of `module.exports`
for consistency with other modules.

Also update documentation in `service-context.js` to use ES exports.

Separate out icon definitions from `SvgIcon` component

Improve documentation per PR feedback.

Validate prop types in tests

Remove remaining implicit jQuery imports

Tighten up display of headers for collapsed replies in threads

Replace the hardcoded set of available icons in `svg-icon.js` with a
`registerIcons` function which is called by the application at startup
to register the icons that should be available for use.

This will enable us to re-use the `SvgIcon` component in the "annotator"
part of the client or other applications (as part of our UI component
library) which use a different set of icons.

The lms application currently has a different solution for this in its
`SvgIcon` implementation which requires the icon markup to be passed
each time the component is used. On reflection, I think that passing
just the name at the point of use is going to be more convenient in app
code and when writing tests.  It also avoids the need to validate the
markup is from a trusted source on each use.

- Remove obsolete comments in `src/sidebar/test/bootstrap.js` about
  Angular using the full version of jQuery. This no longer applies.

- Remove the last few uses of the `$` global variable to reference
  jQuery and replace it with an explicit import. This will make it
  easier to identify the remaining parts of the app that are still using
  it.

This adds basic runtime verification that mocked components are called
with types that are compatible with what the real component expects.

This turned up one existing issue with use of `MenuSection` by
`UserMenu`, where conditional rendering would cause the prop type check
to fail.

For consistency with other tests, use `mockImportedComponents` so that
an issue with an imported component doesn't cause a cascade of failures
in the unit tests for these components.

Only create one highlight element for adjacent text nodes

Replace angular-mocks in service tests

Bump @sentry/browser from 5.15.0 to 5.15.4

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.15.0 to 5.15.4.
- [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/5.15.0...5.15.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump autoprefixer from 9.7.4 to 9.7.5

Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 9.7.4 to 9.7.5.
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/autoprefixer/compare/9.7.4...9.7.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump browserify from 16.5.0 to 16.5.1

Bumps [browserify](https://github.com/browserify/browserify) from 16.5.0 to 16.5.1.
- [Release notes](https://github.com/browserify/browserify/releases)
- [Changelog](https://github.com/browserify/browserify/blob/master/changelog.markdown)
- [Commits](https://github.com/browserify/browserify/compare/v16.5.0...v16.5.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bump dom-seek from 4.0.3 to 5.0.1

Bumps [dom-seek](https://github.com/tilgovi/dom-seek) from 4.0.3 to 5.0.1.
- [Release notes](https://github.com/tilgovi/dom-seek/releases)
- [Commits](https://github.com/tilgovi/dom-seek/compare/v4.0.3...v5.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>