Incrementally expand merge request diffs in both direction

When a diff is collapsed between two hunks, expansion is currently only possible in one direction or the entire file. Also the ellipsis is hard to interpret. We should make the interface clearer and make it possible to expand in either direction.

Further details

It has been raised that the visual distinction between code folds and the source code is actually not strong enough. I expect that this doesn't conform to our accessibility guidelines.

Proposal

Show options to expand up 20, down 20, or all N lines between

Current	Proposed

Final Solution (Explanation here)

Links / references