This PR is part of #4380 - Remove legacy `/api/feature` endpoint.
## About the changes
### Frontend
- Removes the useFeatures hook
- Removes the part of StrategyView that displays features using this
strategy (not been working since v4.4)
- Removes 2 unused features entries from routes
### Backend
- Removes the /api/admin/features endpoint
- Moves a couple of non-feature related tests (auth etc) to use
/admin/projects endpoint instead
- Removes a test that was directly related to the removed endpoint
- Moves a couple of tests to the projects/features endpoint
- Reworks some tests to fetch features from projects features endpoint
and strategies from project strategies
