WIP: Add group wiki support

This MR adds wiki page support to groups. This MR needs to be in WIP while !15060 (closed) is not merged.

Wiki link added to global menu. Wiki for a group has an independent repository. And has the same user behavior as a ProjectWiki (both Project/Wiki and Group/Wiki use the same views (shared/wikis/...)). Permissions for GroupWiki looks the same as ProjectWiki permissions.

Also in this MR I have renamed ProjectWiki (which belonged to Project) to WikiProject (which belongs to Project or Group) Perhaps I need to move this to separate MR (because this is very big)?

Git access to GroupWiki not available now, implementation takes a lot of time for refactoring and I think should be done in other MR later. External wikis support for GroupWiki also is not available, because now external wiki implemented as ProjectService. I think good solution looks like: move external wiki support to features for both Project and Group.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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)