Help & Tutorial 2/n: Add `isSidebar`, `versionData` utilities

Add a utility for organizing information about the current user session
and app version. This will be used by the updated help panel.

The client application has several existing tests to see if the app is
“in sidebar mode” currently. This utility is a first step to harmonizing
this and making things consistent.

Improve accessability for toggle buttons

These don't cover all of the functionality but they do at least cover
the main buttons.

A small change to `toolbar.coffee` was needed so that the button states
are updated correctly when the toolbar is not added to `document.body`.

Help & Tutorial 1/n: Add and update SVG icons

- Add aria attribute for show / hide side bar
- Add aria attribute for show / hide highlights
- Fix a bug that prevents the correct class from showing on the highlight icon when highlights are initially disabled

Remove unroll and convert its use cases to use forEach instead

`unroll()` is not necessary to maintain as it didn't offer any value when writing tests over a simple `forEach()` call over a list of test parameter sets for a common testing function.

Make it easier to test client from a different device

Bump preact from 10.0.0 to 10.0.1

Make it easier to test the development client from a different device by removing
the need to customize the `SIDEBAR_APP_URL` and `PACKAGE_SERVER_HOSTNAME`
environment variables first.

Instead of hardcoding the client/h dev server hostname and scheme in the client's
boot script, support a very simple URL templating scheme where
references to `{current_host}` and `{current_scheme}` are replaced by
the corresponding elements of the boot script's URL.

This means that for example, if the embed script is accessed as
https://Roberts-MacBook-Pro.local:5000/embed.js then the boot script
will use asset URLs such as
https://Roberts-MacBook-Pro.local:3001/assets/sidebar.bundle.js. If
the boot script is accessed as http://localhost:5000/embed.js then
it will generate http://localhost:3001/assets/sidebar.bundle.js
instead.

A related change in h will remove the need to set the `CLIENT_URL` env
var and modify the OAuth client's redirect URL. At present, those steps
are still required.

Bump mocha from 6.2.1 to 6.2.2

Bump @sentry/browser from 5.7.0 to 5.7.1

Bump karma from 4.3.0 to 4.4.1

Bumps [dompurify](https://github.com/cure53/DOMPurify) from 2.0.6 to 2.0.7.
- [Release notes](https://github.com/cure53/DOMPurify/releases)
- [Commits](https://github.com/cure53/DOMPurify/compare/2.0.6...2.0.7)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [karma](https://github.com/karma-runner/karma) from 4.3.0 to 4.4.1.
- [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/v4.3.0...v4.4.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.7.0 to 5.7.1.
- [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/5.7.0...5.7.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [preact](https://github.com/preactjs/preact) from 10.0.0 to 10.0.1.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.0.0...10.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.549.0 to 2.553.0.
- [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/v2.549.0...v2.553.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [mocha](https://github.com/mochajs/mocha) from 6.2.1 to 6.2.2.
- [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/v6.2.1...v6.2.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Convert markdown editor 3/3 - Convert editor and improve toolbar UX

Move the utility function to the top of the file so that all the
components are grouped together.

Also reformat `markdown-editor-test.js`.

 - Remove `user-select` rule. This is no longer needed because the
   toolbar buttons are `button` elements rather than `i` elements
   (which have selectable text)
 - Simplify border rules
 - Use CSS color name instead of a variable for consistency within the
   file

This has been replaced by the `MarkdownView` and `MarkdownEditor`
components.

Convert the editor part of the `<markdown>` component to Preact.

 - Remove the `readOnly` input as this mode is now handled by the
   separate `MarkdownView` component
 - Improve toolbar a11y by:
   - Adding the "toolbar" ARIA role and an aria-label to the toolbar container
   - Using `button` elements for toolbar buttons
   - Increasing the hit target area for buttons
 - Improve toolbar styling by:
   - Using flexbox
   - Using the modern color palette
   - Fixing the padding around the preview output

Convert markdown editor 2/3 - Convert markdown viewer

Convert markdown editor 1/3 - Remove "enabled" state from excerpt component

Convert the view mode of the markdown editor (when `readOnly` is `true`) to Preact.
The edit mode (when `readOnly` is `false`) will become a separate
component.

Simplify the `<excerpt>` component by removing the `enabled` property.
This property was used to avoid the markdown editor being truncated when
rendered inside the excerpt. However it is possible to achieve the same
result without this property by using `ng-if`.

This change will simplify porting the excerpt component to Preact and
also paves the way to splitting the existing `<markdown>` component into
two separate, simpler, components: One for the "view" mode and one for the "edit"
mode.

Bump @sentry/browser from 5.6.3 to 5.7.0

Revert "Improve accessability for toggle buttons"

This reverts commit ab3d902f.

This change caused the client to fail to load on pages where
`window.hypothesisConfig` is not defined.