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
```

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

Consolidate font sizes and line heights

Add `make sure`

Bump mocha from 7.2.0 to 8.0.1

Bump puppeteer from 3.3.0 to 4.0.0

Bump karma from 5.0.9 to 5.1.0

Bump eslint-plugin-mocha from 5.3.0 to 7.0.1

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.691.0 to 2.696.0.
- [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/v2.691.0...v2.696.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 17.11.0 to 18.0.0.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v17.11.0...v18.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [mocha](https://github.com/mochajs/mocha) from 7.2.0 to 8.0.1.
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mochajs/mocha/compare/v7.2.0...v8.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 3.3.0 to 4.0.0.
- [Release notes](https://github.com/puppeteer/puppeteer/releases)
- [Commits](https://github.com/puppeteer/puppeteer/compare/v3.3.0...v4.0.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [karma](https://github.com/karma-runner/karma) from 5.0.9 to 5.1.0.
- [Release notes](https://github.com/karma-runner/karma/releases)
- [Changelog](https://github.com/karma-runner/karma/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma/compare/v5.0.9...v5.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [eslint-plugin-mocha](https://github.com/lo1tuma/eslint-plugin-mocha) from 5.3.0 to 7.0.1.
- [Release notes](https://github.com/lo1tuma/eslint-plugin-mocha/releases)
- [Changelog](https://github.com/lo1tuma/eslint-plugin-mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/lo1tuma/eslint-plugin-mocha/compare/5.3.0...7.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Fixes https://github.com/hypothesis/client/issues/2177

Add utility class for primary-styled button

Needed by the “Experimental New Note Button”

Fix rendering issue when the org logo is missing by omitting the <img> entirely 

There was a `font-size` rule legacy from (presumably) when the logo was
via an icon font.