Show decorations on diff code views

Created by: felixfbecker

Will fix #911

• Fix perf problems, see #3284 #3285
• Implement getLineElementFromLineNumber() for all code hosts
• Implement diff decorations for the Sourcegraph webapp
• Fix decorations not getting cleaned up
• Add test
• Change Codecov extension to not use transparent colors. The colors must have 100% alpha, or the diff colors will shine through, which makes it impossible to read. They should define 100% alpha colors, with different colors for light and dark. https://github.com/codecov/sourcegraph-codecov/pull/30

Possible optimization: getCodeElementFromLineElement instead of getCodeElementFromLineElement()

Outdated screenshots (background color is now applied to the whole line if Decoration.isWholeLine === true):

Old Screenshots

Test Plan

Code Hosts

• GitHub
• GitHub Enterprise
• Refined GitHub
• Phabricator
• Phabricator integration
• Bitbucket
• Gitlab

Browsers

• Chrome
• Firefox

Created by: codecov[bot]

Codecov Report

Merging #3288 into master will increase coverage by 0.1%. The diff coverage is 81.21%.

Impacted Files	Coverage Δ
browser/src/libs/code_intelligence/code_views.ts	80.76% <ø> (+7.69%)
.../code_intelligence/code_intelligence_test_utils.ts	94.89% <100%> (+0.57%)
browser/src/shared/util/dom.tsx	68% <100%> (ø)
browser/src/libs/bitbucket/code_intelligence.tsx	67.18% <100%> (ø)
...r/src/libs/code_intelligence/code_intelligence.tsx	70.37% <100%> (+4.62%)
shared/src/api/client/services/decoration.ts	75.86% <12.5%> (-24.14%)
browser/src/libs/github/dom_functions.ts	70.23% <67.74%> (+4.44%)
browser/src/libs/bitbucket/dom_functions.ts	63.82% <75%> (+1.6%)
browser/src/libs/code_intelligence/extensions.tsx	82.6% <78.26%> (+3.24%)
browser/src/libs/phabricator/dom_functions.ts	66.66% <78.26%> (+1.09%)
... and 81 more

Created by: todo[bot]

this does not handle decorations that span multiple lines

https://github.com/sourcegraph/sourcegraph/blob/d763780c9d28301ac0f2760642d661357bc6f5fe/shared/src/api/client/services/decoration.ts#L80-L85

---

This comment was generated by todo based on a @todo comment in d763780c9d28301ac0f2760642d661357bc6f5fe in #3288. cc @sourcegraph.

Created by: felixfbecker

Ready for review