Manually delete unused keys from redis activity cache

[mouse-reeve]

This adds an admin UI to delete keys from the redis activity cache that are no longer in use.

Remaining work:

• Separate out the type of keys to delete (cache vs activitystreams)
• Allow deletion of entire django cache, separately from cache items with an outdated prefix
• Ensure that the activitystreams code isn't re-creating invalid keys (ie, for deleted users)
• [ ] Maybe add a schedule-able task to remove unused keys?

Screenshots

When you open the page:

When you scan for outdated keys:

If you want to clear the Django cache: