Add Rank details to army
Import specific models from django_phpBB, not the whole module
Model tweaks for Player and Campaign

Player:

* Make drafted_for a one-to-many relationship.
* Change related_name for relation to phpbb_user.

Campaign:

* Add a draft_enabled boolean flag
Recreate migrations

The migration for Player.drafted_for from a many-to-many to a
one-to-many relation doesn't work automatically cause there
could be several relations added to one player in the database.

We don't care at this point so we start over.