Implement Diff File header

We should implement this part of the Diff in Rapid Diffs:

Discussions should be done as a part of this epic: Implement Discussions in Rapid Diffs (&15539).

Viewed state should be done in Implement viewed state for diff file in MRs (#499293).

Collapsing and expanding viewed file should be done in Implement file collapsing/expanding for viewed ... (#498891).

Update: ellipsis menu with option to View file @ (sha) and Edit in single file editor are part of this issue.