1578 active in category

[davidmiller]

This is the final PR which closes #1578

Note that setting the value of .active in save without looking at the original value from the database could have resulted in this:

episode = Episode.objects.get(13)
episode.active = False
episode.save()
episode.active
# -> True

Instead we try to catch this and raise an exception to alert the user - the correct way to do this is to
have a category that agrees with whatever condition meant you thought you should set .active to False.