Introduces a new `diff` action on the wiki controllers which shows
the diff for the selected file and commit.

- Add `Gitlab::Diff::FileCollection::WikiPage`.
- Override diff helpers in wiki controller.
- Unify wiki views:
  - Always use limited container width.
  - Always add breadcrumbs for wiki page path.
  - Show sidebar in diff view.
  - Align title in diff view with history view.
  - Add helpers for wiki page headers.