Add method missing from EE

What does this MR do?

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

No

Why was this MR needed?

ProjectWiki#repository_exists? is present in EE but not on CE, and there's no particular reason why it shouldn't

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• 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