Remove unused variable
Models: Remove slug; remove title/desc from player and Participation

* Remove slug entirely
* Move title/description into a seperate mixin and include them in everything
  but Player and CampaignParticipation
admin: Customize player form

* Move registrations to abcapp.admin
* Make a custom admin form for Player that
  * inlines the CampaignParticipation model
  * denies creating/deleting players
  * makes the FK to phpbb_user and django_user read only