Fix menu arrow alignment and overlap with content

The menu arrow was previously implemented with a 45-degree rotated
square with border on two sides and white background. This had two
problems:

 - Because a square's diagonal is longer than its width, the right edge
   after rotation extended beyond the right edge of the content

 - The bottom part of the rotated square overlapped the menu content,
   which created a "notch" in the top menu item when hovered

Fix the problem and simplify the CSS by creating the triangle shape
using an SVG instead.

Convert timestamp component to React

- Use now variable instead of walking the clock.
- Use iso format instead of seconds.
- Pass a Date object instead of an iso format to time.<methods>so
the methods off the Date object can be mocked in order to avoid
timezone specific tests. Since methods like getFullYear output the
year in whatever timezone the operating system is set to, these
methods must be mocked/mapped to their UTC equivalents when testing
such as getUTCFullYear.
- Remove offseting of time based on timezone as this is now covered
by mocking the getFullYear method and not needed for the decayInterval
as that only cares about time deltas.

Bump @octokit/rest from 16.27.0 to 16.27.3

Bump gulp-changed from 3.2.0 to 4.0.0

Bumps [gulp-changed](https://github.com/sindresorhus/gulp-changed) from 3.2.0 to 4.0.0.
- [Release notes](https://github.com/sindresorhus/gulp-changed/releases)
- [Commits](https://github.com/sindresorhus/gulp-changed/compare/v3.2.0...v4.0.0)

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.27.0 to 16.27.3.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.27.0...v16.27.3)

Replace several uses of `console.warn` with `warnOnce`

The stub is re-initialized for every test, so there is no point in
checking at the start.

This reduces warning spam in the event where the code path that triggers
the warning is invoked repeatedly and there is no value to the developer
in telling them about the issue more than once.

To make `warnOnce` an easier drop-in replacement for `console.warn`, it
has been modified to accept multiple arguments, all of which get
forwarded to `console.warn`.

Upgrade Preact

Move firstKey function from sidebar-content to store

Add utility to re-render components when store changes

 - Simplify an example. `useCallback` is not required.
 - Add a comment regarding effect cleanup

This moves the firstKey function from the sidebar-content into
the selection store module.

Fix an issue where `GroupListItem` components did not re-render when the
focused group changed. This component is due to be replaced shortly, so
this is really just a test case to show that `useStore` works as
expected.

Add a hook that connects a store to a React/Preact component and ensures that
it re-renders when relevant store state changes.

Merge pull request #1141 from hypothesis/dependabot/npm_and_yarn/babel-plugin-transform-async-to-promises-0.8.11

Bump babel-plugin-transform-async-to-promises from 0.8.10 to 0.8.11

This fixes a code generation bug which broke the latest version of
babel-plugin-transform-async-to-promises.

Bump @octokit/rest from 16.26.0 to 16.27.0

Bump raven-js from 3.27.0 to 3.27.1

Bump express from 4.17.0 to 4.17.1

Bump @babel/core from 7.4.4 to 7.4.5

Bumps [babel-plugin-transform-async-to-promises](https://github.com/rpetrich/babel-plugin-transform-async-to-promises) from 0.8.10 to 0.8.11.
- [Release notes](https://github.com/rpetrich/babel-plugin-transform-async-to-promises/releases)
- [Commits](https://github.com/rpetrich/babel-plugin-transform-async-to-promises/compare/v0.8.10...v0.8.11)

Bump @babel/preset-env from 7.4.4 to 7.4.5

Bump core-js from 2.6.6 to 2.6.9

Bumps [express](https://github.com/expressjs/express) from 4.17.0 to 4.17.1.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](https://github.com/expressjs/express/compare/4.17.0...4.17.1)

Bumps [raven-js](https://github.com/getsentry/raven-js) from 3.27.0 to 3.27.1.
- [Release notes](https://github.com/getsentry/raven-js/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/raven-js/commits)

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 16.26.0 to 16.27.0.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v16.26.0...v16.27.0)

Bumps [@babel/core](https://github.com/babel/babel) from 7.4.4 to 7.4.5.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.4.4...v7.4.5)

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.4.4 to 7.4.5.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/compare/v7.4.4...v7.4.5)

Bumps [core-js](https://github.com/zloirock/core-js) from 2.6.6 to 2.6.9.
- [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/compare/v2.6.6...v2.6.9)

[Security] Bump tar from 2.2.1 to 2.2.2