Replace legacy Sentry JS SDK with new Sentry SDK

This enables us to distinguish reports from dev, qa, prod etc. The value
is provided by h for the embed or the browser extension.

Previous experience with Sentry has found that there are various
scenarios that can cause a client to spam the server with a large number
of reports. Although Sentry has its own server-side quotas and rate
limiting, I think it will be useful to do client-side per-session rate
limiting as well. This also provides a place where we can hook in other
client-side filtering in future.

Replace the legacy raven-js package with Sentry's new JS SDK [1], with a
minimal wrapper in `util/sentry.js` that exports the same interface as
the previous `raven.js` module.

To enable Sentry, h or the browser extension must set a `sentry` key in the
sidebar app's configuration.

As part of switching SDKs, the following changes were made:

 - The AngularJS integration was removed, as we are in the process of
   migrating away from Angular
 - The custom code for unhandled promise rejections was removed, as the
   new SDK handles this already
 - The URL rewriting logic was removed. This was done to make the setup
   as minimal as possible. This may need to be re-added later.
 - The `report` function was removed, as it was not used anywhere
 - The `session` module now imports `util/sentry` directly rather than
   using Angular dependency injection. Now that we have a convenient way
   to mock all CJS/ES imports, there isn't a need to use DI for
   utilities

[1] https://docs.sentry.io/platforms/javascript/

Namespace the frames module

Namespace the direct-linked module

Namespace the activity module

Namespace the drafts module

Add namespace capability to store modules

Each module can now opt into being namespaced by exporting the value for "namespace".

Bump katex from 0.10.2 to 0.11.0

Bump gulp-changed from 4.0.0 to 4.0.1

Bump karma-chrome-launcher from 2.2.0 to 3.1.0

Bumps [eslint](https://github.com/eslint/eslint) from 6.1.0 to 6.2.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v6.1.0...v6.2.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [karma-chrome-launcher](https://github.com/karma-runner/karma-chrome-launcher) from 2.2.0 to 3.1.0.
- [Release notes](https://github.com/karma-runner/karma-chrome-launcher/releases)
- [Changelog](https://github.com/karma-runner/karma-chrome-launcher/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma-chrome-launcher/compare/v2.2.0...v3.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [gulp-changed](https://github.com/sindresorhus/gulp-changed) from 4.0.0 to 4.0.1.
- [Release notes](https://github.com/sindresorhus/gulp-changed/releases)
- [Commits](https://github.com/sindresorhus/gulp-changed/compare/v4.0.0...v4.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [katex](https://github.com/KaTeX/KaTeX) from 0.10.2 to 0.11.0.
- [Release notes](https://github.com/KaTeX/KaTeX/releases)
- [Changelog](https://github.com/KaTeX/KaTeX/blob/master/CHANGELOG.md)
- [Commits](https://github.com/KaTeX/KaTeX/compare/v0.10.2...v0.11.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add the experimental focus feature to the docs

Fix JS error when searching after creating a page draft

This adjusts view-filter so it can handle draft annotations which can be defined by a missing id.

`npm` is still used by the final deployment process to publish packages
to the npm registry.

Switch default browser for running tests from PhantomJS to headless Chrome

These are no longer needed when running tests under Chrome.

Update our test/CI environment to a modern browser which matches what
our users use and supports modern web APIs and ES language features.

 - Add a Dockerfile which defines an image that includes everything
   needed to run client tests and deployment steps. This image uses
   plain Alpine as the base rather than Debian as this makes it easier
   to update frequently (every 6 months) to get a current Chrome build.

 - Replace PhantomJS with Puppeteer for testing locally and on Travis.
   Jenkins uses Chromium packages from Alpine.

- Fix failing `services/api.js` test

  Chrome throws an error when trying to construct a fetch `Response` with
  a non-null `body` if the status is 204.

- Fix Timestamp and GroupListItem test failures in Chrome

  - Pass required type argument to `Event` constructor
  - Pass valid `timestamp` argument to `Timestamp`. This value is passed
    on to date functions which require a valid date/time string.

- Fix Slider transition test in Chrome

  Since the transition from open to closed is animated, the actual element
  height doesn't immediately transition to 0px. Instead of checking the
  actual height, check the target height specified by the current style
  properties.

  The previous code happened to work under PhantomJS, but was conceptually incorrect.

Change focus mode button label

Change the label when showing everyone's annotations from "Only <name>"
to "Show only <name>". Having the label always start with a verb is more
consistent and hopefully a bit clearer.

Remove "X" icon from toggle button in focus mode

Since clicking the button toggles between two states rather than
dismissing the "focus mode" header, the "X" icon doesn't feel
appropriate, so remove it.

The button's padding has been adjusted to center align the content
following the removal of the icon.

Fixes #1294

Add first iteration of UI to user-focused mode

Bump sinon from 7.3.2 to 7.4.1