assigns migration steps to User database objects

adds the migration_handler to the User db class, connecting the
migration steps in ../db/migrations.py to the migration code in
gmg_commands

mediagoblin/db/models.py

@@ -58,6 +58,8 @@ class User(Document):
         'status': u'needs_email_verification',
         'verification_key': lambda: unicode(uuid.uuid4()),
         'is_admin': False}
+        
+    migration_handler = migrations.UserMigration
 
     def check_login(self, password):
         """