Single file diffs

Closes #14103. Related (not part of this MR): #19553.

This adds a `diff_for_path` action to each place we render diffs (commit, compare, new MR, existing MR) which renders the diff for a single path. The action is always available with the same params as the 'parent' action, to make it simpler to generate the URIs.

If a diff is bigger than 10 KB, it will be collapsed by default and have a data attribute added. You can then click the message or the filename to expand that diff. For expanded files, you can collapse and expand them, but they won't make any AJAX requests.

![Expand_and_collapse_diffs](/uploads/a4072029085082b85c47006f67ac531c/Expand_and_collapse_diffs.gif)

See merge request !4990