[MVC 1] Group-level wiki - Support one wiki at group level
Below is the work required as moved over from the Technical research issue: https://gitlab.com/gitlab-org/gitlab/issues/29798
## BE Proposal Summary
We plan to:
1. Verify the Projects has been decoupled
2. Refactor Models, Services, Controllers, Repositories
3. Refactor current Wiki permissions to be supported at the Group Level
4. Updates for routing
## FE Proposal Summary
There will be minimal Frontend work for the first MVC for Group Level Wiki. Only a wiki link to the left side navigation is needed.
## Feature Flag
Any functionality that's exposed to users needs to be behind a group-level feature flag called `:group_wikis`.
The feature flag rollout is tracked in https://gitlab.com/gitlab-org/gitlab/-/issues/207888.
## Testing
- Since we're adapting the existing project wikis functionality to the group-level, we can reuse the existing specs and refactor them to test both project and group wikis.
- Subgroups generally shouldn't require any special treatment, but everywhere we deal with group paths we should include tests for subgroups as well.
## Next steps
- [x] Move each bullet to issue (@cdybenko)
- [x] BE weights (TBD) @dsatcher
- [x] FE weights (3) @rkuba
- [x] Solidify order of operations Frontend and Backend and re-order below
- [x] Assign Milestones from 12.9 - 13.0
epic