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Outdated screenshots (background color is now applied to the whole line if Decoration.isWholeLine === true):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