settings for field/word colors in the image.

* settings for the colors in the generated images
* added veto color in the marked image
changed fields to readonly in the admin interface

* many fields should not be changed anyway
* generating a list of possible Games for a BingoBoard or BingoBoards
  for a BingoField takes a long time, if there are many such objects
  from old games.
use select_related on the fields queryset
removed hardcoded timezone
minor: some text changes
check if game is not None before calling game.save()
added german translation