Resolve - Cannot diff changes in wiki history

What does this MR do?

Adds ability to diff changes to wiki pages.

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

app/models/project_wiki.rb re-assigns the commit's project (line 136). A bit of a hack.

Why was this MR needed?

Adds a highly requested feature for my user base.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/17906

Screenshots (if relevant)

Updates to wiki page history:

New wiki page diff:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [na] Documentation created/updated
• [na] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together Closes #17906 (moved)