In advance of some migration work, remove some unused classes, extract
some color variables and organize.

Update Cancel UI for `AnnotationPublishControl`

Don't filter annotations when showing "All Students"

When the client is running in the LMS app and in grading mode, don't
apply a broken filter to the annotations list when the LMS app is asking
the client to show "All Students".

Fixes https://github.com/hypothesis/lms/issues/1259

The LMS app sends username not userid:

https://github.com/hypothesis/lms/blob/d6b88fd7e375a4b23899117556b3e39cfe18986b/lms/static/scripts/frontend_apps/components/LMSGrader.js#L36

But the test for ths client code that receives this was sending
userid.

Fix the test to send username, the same as the LMS app does.

Consolidate all grey colors across client application

Migrate usages of `$gray-light` to `$grey-semi`

Migrate usages to `$grey-5`

Introduce a new color variable, `$grey-mid`, which sits between
`$color-5` and `$color-6` in the current palette scale and fills a
large gap.

Remove `$gray-dark` and `$color-dove-gray` after migrating to `$grey-mid`.

Migrate single usage to `$grey-5`

Migrate all usages to `$grey-6` (close approximate).

Migrate `$gray-lighter` SASS color to `$grey-3`

`$gray-lighter` is deprecated. Its color value (#d3d3d3) is close enough
to `$grey-3` (#dbdbdb) to make a simple substitution feasible.

Remove unused styles referencing `$gray-lighter`

Update core-js to current version

Replace deprecated gulp-util dependency

Bump @sentry/browser from 5.10.1 to 5.10.2

Bump preact from 10.0.5 to 10.1.0

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.10.1 to 5.10.2.
- [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/5.10.1...5.10.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.585.0 to 2.586.0.
- [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/v2.585.0...v2.586.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [preact](https://github.com/preactjs/preact) from 10.0.5 to 10.1.0.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.0.5...10.1.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.7.4 to 7.7.6.
- [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/v7.7.4...v7.7.6)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

This package provides polyfills for new ES/DOM APIs for use in IE 11.
This was held back to a v2.x version due to a conflict with fetch-mock
that is now resolved.

The only function we use from gulp-util is `log`, which is replaced by
fancy-log per the migration guidelines at https://medium.com/gulpjs/gulp-util-ca3b1f9f9ac5.

Bump gulp-rename from 1.4.0 to 2.0.0

Bumps [gulp-rename](https://github.com/hparra/gulp-rename) from 1.4.0 to 2.0.0.
- [Release notes](https://github.com/hparra/gulp-rename/releases)
- [Changelog](https://github.com/hparra/gulp-rename/blob/master/CHANGELOG.md)
- [Commits](https://github.com/hparra/gulp-rename/commits)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Remove unused `gulp-{if, postcss, sourcemaps}` dependencies

Bump fetch-mock from 8.0.0 to 8.0.1

Bump core-js from 2.6.10 to 2.6.11

Bump websocket from 1.0.30 to 1.0.31

Bump chance from 1.1.3 to 1.1.4

Bump postcss from 7.0.23 to 7.0.24

Bump terser from 4.4.0 to 4.4.2

Bump @sentry/browser from 5.9.1 to 5.10.1

Bumps [@babel/core](https://github.com/babel/babel) from 7.7.4 to 7.7.5.
- [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/v7.7.4...v7.7.5)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>