Hide Gollum inside Gitlab::Git::Wiki

What does this MR do?

Moves all code that touches Gollum (Git-based wikis) into Gitlab::Git::Wiki and related classes.

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

Why was this MR needed?

We want to run Gollum inside gitaly-ruby.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?