Fix file diff title and renames alignment in discussions

What does this MR do?

• Fix file diff title and renames alignment in discussions
  • Use similar styles to the actual diff changes tab, see https://gitlab.com/gitlab-org/gitlab-ce/blob/43e1481c8191651880fe20ff2b07a5501a5f7112/app/views/projects/diffs/_file.html.haml#L5

Before	After

Are there points in the code the reviewer needs to double check?

• The unused toggle icon (invisible in the screenshot but adds space) was removed in this commit previously, https://gitlab.com/gitlab-org/gitlab-ce/commit/cc37d1dfa11691f2d8357151d4b722b3d114881e#cf11229dd714f7f4b46f102ce6846e25528b6689_7_6

Why was this MR needed?

• Unify file diff titles with "Changes" tab, UI polish

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31782 (closed)