migrate to per-model UUID Primary key, remove UUIDModel (#26)

* *: migrate to per-model UUID Primary key, remove UUIDModel

* *: fix import order, fix unittests

passbook/lib/models.py

@@ -1,6 +1,4 @@
 """Generic models"""
-from uuid import uuid4
-
 from django.db import models
 
 
@@ -12,12 +10,3 @@ class CreatedUpdatedModel(models.Model):
 
     class Meta:
         abstract = True
-
-
-class UUIDModel(models.Model):
-    """Abstract base model which uses a UUID as primary key"""
-
-    uuid = models.UUIDField(primary_key=True, editable=False, default=uuid4)
-
-    class Meta:
-        abstract = True