django/models: Ensure OIDCUser model will not be created in database

OIDCUser model will not be persisted to django database as those
information are already stored in the identity provider one.

So ensure no table for the model will be created in django database
by setting the auto_created meta flag to True.

Related to T3150

swh/auth/django/models.py

@@ -41,6 +41,7 @@ class OIDCUser(User):
         # and isn't in an application in INSTALLED_APPS"
         app_label = ""
         proxy = True
+        auto_created = True  # prevent model to be created in database by migrations
 
     def save(self, **kwargs):
         """