Adds a `features` app with a swappable waffle Flag (WAFFLE_FLAG_MODEL) that
gains a m2m to Club, so a feature can be rolled out club by club.
Two things worth calling out:
- `everyone` keeps waffle's contract of overriding *all* other targeting, so
club targeting is only consulted when `everyone is None`. This keeps
`everyone = False` usable as a hard kill-switch.
- m2m edits don't call save(), so waffle's per-flag cache would go stale when
clubs are added or removed. A m2m_changed receiver flushes it from both
directions, and get_flush_keys() drops the club-set key alongside waffle's own.
Note for future flag tests: waffle's cache is not rolled back with the test
transaction, so tests touching flags must clear it (see features/tests.py).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
