Clean up Gitlab::Git::Wiki classes and reduce use of Gollum

What does this MR do?

These classes were shared between gitlab-ce and gitaly-ruby for some time. That is no longer the case so we can simplify them, and correct some of the comments.

This change also reduces how much we load of gollum-lib. Gollum-lib is almost entirely encapsulated in Gitaly now, with the exception of Gitlab::Git::Wiki#preview_slug which runs in gitlab-ce.

As a side effect of not loading all of gollum-lib it became apparent that we directly rely on the github-markup gem without explicitly requiring it in the Gemfile. I added it to the Gemfile to rectifiy this.

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

Why was this MR needed?

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
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?