Tying LOT_TYPE_CHOICES to the model creates impossible migrations

[aidanlister]

Specifying LOT_TYPE_CHOICES as the choices for the LOT.type field means that migrations will be created for any types created -- however the migrations are created against the contributed module which means they're not going to be included in source control, which means they aren't being deployed.

Let's not do that and just have a validate() method.