Pin Celery to 3.0.19
Default-disable database admin.

- Too many people shooting themselves in the foot mucking with the
  database directly
- There are now built-in forms for performing "safe" edits in the
  admin dashboard
- Should be visible motivation to finish them off by next release :)
- Can re-enable with KEGBOT_ENABLE_ADMIN = True in local_settings.
core: fix short drink redirect; add test coverage.

Closes #130.