Update base clases for extensible models

ExtensibleModel is now an abstract base model.

aleksis/core/migrations/0001_initial.py

@@ -26,7 +26,7 @@ class Migration(migrations.Migration):
             options={
                 'ordering': ['short_name', 'name'],
             },
-            bases=(models.Model, aleksis.core.mixins.ExtensibleModel),
+            bases=(aleksis.core.mixins.ExtensibleModel,),
         ),
         migrations.CreateModel(
             name='School',
@@ -98,7 +98,7 @@ class Migration(migrations.Migration):
             options={
                 'ordering': ['last_name', 'first_name'],
             },
-            bases=(models.Model, aleksis.core.mixins.ExtensibleModel),
+            bases=(aleksis.core.mixins.ExtensibleModel,),
         ),
         migrations.AddField(
             model_name='group',

aleksis/core/models.py

@@ -64,7 +64,7 @@ class SchoolTerm(models.Model):
         verbose_name_plural = _("School terms")
 
 
-class Person(models.Model, ExtensibleModel):
+class Person(ExtensibleModel):
     """ A model describing any person related to a school, including, but not
     limited to, students, teachers and guardians (parents).
     """
@@ -159,7 +159,7 @@ class Person(models.Model, ExtensibleModel):
         return self.full_name
 
 
-class Group(models.Model, ExtensibleModel):
+class Group(ExtensibleModel):
     """Any kind of group of persons in a school, including, but not limited
     classes, clubs, and the like.
     """