Show past comments from diffs on file

Problem to solve

When I explore a codebase, it's not always clear why certain decisions were made. Going from a single file to any line comments that were made in MRs requires a lot of clicks (file, blame, commits, merge request, diff), even though those comments / discussions can give me valuable insights and context.

Further details

This would be useful for anyone exploring any file, in the file view or the Web IDE.

This feature might be significantly more used if we'd allow offline code review: https://gitlab.com/gitlab-org/gitlab-ee/issues/1769

See how codestream does this: https://codestream.com/

Proposal

If any comments are made on a particular line of a file, show a marker that indicates this. Clicking on that would bring you or show you the discussion that was had over that line.

What does success look like, and how can we measure that?

This would help people gain a better understanding of code. That's hard to measure.

We should see an increase in visits to closed merge requests though.