[Tech Debt] Explore alternative ways of handling sticky headers at the top of the diffs tab on merge requests

Right now we are stacking multiple sticky headers on the diffs tab of the merge request page: The MR-Tabs (Discussions, commits, pipelines, diffs), the Compare Versions header, and the file headers. This requires a lot of absolute positioning, and makes it difficult to change the height of anything. One reason we may want to change the height is to stack information when we have too many buttons in one of the headers.

This issue is to explore alternatives for these sticky headers so we can better style things in the future.