WIP: Add group wiki support

This MR adds wiki page support to groups.

General workflow looks like:

• on group settings page, user can select Wiki Project. In this dropdown exist all group projects with enabled wiki.
• on group page user sees Wiki tab
• on this tab user sees wiki from selected project. All wiki behavior occurs in group context (all links behave as group links)

I'm not sure in dropdown on group settings page (I like dropdowns from MR page)

Tests and docs not implemented/fixed yet.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Changes for admin area
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #4037 (moved)