Ports changes to models and model tests along with some settings changes...

... from feature/tracks_activation
It looks like there is a many-to-many relationship between Stages and Tracks that run through the StageUrl join table.  Instead of going through the Track and StageUrl models here, why not create a ManyToMany field on the Track model?  Something like: `stages = models.ManyToManyField(Stage, through='StageUrl').`  This is a useful convention that Django provides for just such relationships - more details here https://docs.djangoproject.com/en/dev/topics/db/models/#extra-fields-on-many-to-many-relationships