Fix the moderation_queue.models.QueuedImage why_allowed choices attribute

[mhl]

Currently QueuedImage has a CharField that has a choices attribute which contains a reference to 'Democracy Club'

• https://github.com/mysociety/yournextrepresentative/blob/master/moderation_queue/models.py#L31-L50

Unfortunately if you change the choices field, Django's migrations code wants you to generate a migration because it's a model change, even though it wouldn't actually change the database:

• https://code.djangoproject.com/ticket/22837

Maybe the choices should only be only defined on a corresponding form class or the field should be a foreign key to a model that defines the reasons a photo might be allowed.