Begin to add JSDoc types for client configuration objects

The client's "sidebar" and "annotator" applications query configuration
from various sources (the host page, the H backend / browser extension
build, added dynamically by the app).

This commit begins the process of adding a centralized place to document
the types of these configuration objects and making typechecked modules
in `src/sidebar/util` use them.

Add command to typecheck code and enable it for src/sidebar/util/

 - Ignore an unused function
 - Use a more modern idiom for setting the default value for a test
   seam

Reading the code it appeared that `getBreakpoint` could return `null`,
but this was not actually the case because the last item in
`BREAKPOINTS` always matches.

Revise the way `getBreakpoint` works to make it more obvious that it
will always return a breakpoint.

Bumps [eslint-plugin-jsx-a11y](https://github.com/evcohen/eslint-plugin-jsx-a11y) from 6.2.3 to 6.3.1.
- [Release notes](https://github.com/evcohen/eslint-plugin-jsx-a11y/releases)
- [Changelog](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/blob/master/CHANGELOG.md)
- [Commits](https://github.com/evcohen/eslint-plugin-jsx-a11y/compare/v6.2.3...v6.3.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>
Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [fetch-mock](https://github.com/wheresrhys/fetch-mock) from 9.10.1 to 9.10.2.
- [Release notes](https://github.com/wheresrhys/fetch-mock/releases)
- [Commits](https://github.com/wheresrhys/fetch-mock/compare/v9.10.1...v9.10.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>
Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [terser](https://github.com/terser/terser) from 4.7.0 to 4.8.0.
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/terser/terser/compare/v4.7.0...v4.8.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>
Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [axe-core](https://github.com/dequelabs/axe-core) from 3.5.4 to 3.5.5.
- [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/v3.5.4...v3.5.5)
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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Bumps [sass](https://github.com/sass/dart-sass) from 1.26.8 to 1.26.9.
- [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/1.26.8...1.26.9)
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Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 9.8.0 to 9.8.2.
- [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/9.8.0...9.8.2)
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Standardize font mixins

Extract "molecule" compositions into reusable mixins

Log which tests have executed during CI runs

During CI runs, but not local development, log which individual tests
have executed. This may help with debugging https://github.com/hypothesis/client/issues/2249.

Add atomic mixins for borders, icons

Add basic layout mixins

Bumps [@babel/core](https://github.com/babel/babel/tree/HEAD/packages/babel-core) from 7.10.2 to 7.10.3.
- [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/v7.10.3/packages/babel-core)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.696.0 to 2.702.0.
- [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/v2.696.0...v2.702.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.10.2 to 7.10.3.
- [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/v7.10.3/packages/babel-preset-env)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Update Alpine in CI to v3.12

Remove Node v14 workaround for Karma

Update the software used to run tests in CI. This _may_ help with
https://github.com/hypothesis/client/issues/2249.

Run tests under Puppeteer again as the issue with Node v14 has been
resolved.

Fix warning about file type of CoffeeScript test modules

Fix several warnings like this when running tests:

```
22 06 2020 14:01:32.661:WARN [middleware:karma]: Unable to determine
   file type from the file extension, defaulting to js.
     To silence the warning specify a valid type for
     /Users/robert/hypothesis/repos/client/src/annotator/test/sidebar-test.coffee
     in the configuration file.
       See http://karma-runner.github.io/latest/config/files.html
```

In my experience this surfaces a lot of real potential issues and is
harder to turn on later.

Also set the `target` field, as otherwise TS assumes that we're going to
compile down to ES3 (!).

Create mixins for establishing equal horizontal or vertical spacing
between immediate children of an element

Create simple mixins for establishing flex containers to consolidate
layout use of flexbox in client component CSS

Remove redundant CSS rules

Remove CSS rules that “are the case already” via
inheritance.

Remove rules that are overridden and never apply