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>

Bumps [postcss](https://github.com/postcss/postcss) from 7.0.23 to 7.0.24.
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.23...7.0.24)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [fetch-mock](https://github.com/wheresrhys/fetch-mock) from 8.0.0 to 8.0.1.
- [Release notes](https://github.com/wheresrhys/fetch-mock/releases)
- [Commits](https://github.com/wheresrhys/fetch-mock/compare/v8.0.0...v8.0.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [chance](https://github.com/chancejs/chancejs) from 1.1.3 to 1.1.4.
- [Release notes](https://github.com/chancejs/chancejs/releases)
- [Commits](https://github.com/chancejs/chancejs/compare/1.1.3...1.1.4)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [terser](https://github.com/terser/terser) from 4.4.0 to 4.4.2.
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](https://github.com/terser/terser/compare/v4.4.0...v4.4.2)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [core-js](https://github.com/zloirock/core-js) from 2.6.10 to 2.6.11.
- [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/v2.6.10...v2.6.11)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 5.9.1 to 5.10.1.
- [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/5.9.1...5.10.1)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.581.0 to 2.585.0.
- [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/v2.581.0...v2.585.0)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [websocket](https://github.com/theturtle32/WebSocket-Node) from 1.0.30 to 1.0.31.
- [Release notes](https://github.com/theturtle32/WebSocket-Node/releases)
- [Changelog](https://github.com/theturtle32/WebSocket-Node/blob/master/CHANGELOG.md)
- [Commits](https://github.com/theturtle32/WebSocket-Node/compare/v1.0.30...v1.0.31)
Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add charset token to Content-Type header for client files deployed to S3

V3 sourcemaps are JSON objects [1]. Using the `application/json` mime
type makes it more obvious why this is listed under text formats.
Sourcemaps are defined to be UTF-8 so the `charset` token is
superfluous, but it shouldn't cause problems either.

See also https://stackoverflow.com/questions/19911929/

[1] https://sourcemaps.info/spec.html#h.qz3o9nc69um5

Avoid effect in tag list