fix: updating the timestamp of new games works now
last_used is now auto_now_add and updated via .update()

now last_used is NOT updated when a Game/BingoBoard is edited.
This is needed for adding Game descriptions after the Game is finished.
migration: unique_together of game_id and board_id removed, auto_now_add for last_used fields

- unique_together of game_id and board_id is now checked in .save(),
  because it depends on the Site, too.
- last_used is auto_now_add and updated via .update(), because editing
  Game / BingoBoard objects in the admin interface should not change the
  timestamp.