Support line decorations on PRs/diffs

Created by: chrismwendt

Feature request description

Line decorations on PRs and diff pages https://github.com/sourcegraph/sourcegraph/issues/852#issuecomment-437513606

VS Code Screenshot

Is your feature request related to a problem? If so, please describe.

Not really, just the lack of Codecov information on PRs/diffs.

Describe alternatives you've considered.

Viewing each side of the diff as a full code file independently.