Fixed #22634 -- Made the database-backed session backends more extensible.

Introduced an AbstractBaseSession model and hooks providing the option
of overriding the model class used by the session store and the session
store class used by the model.

tests/runtests.py

@@ -140,6 +140,7 @@ def setup(verbosity, test_labels):
         # us skip creating migrations for the test models.
         'auth': 'django.contrib.auth.tests.migrations',
         'contenttypes': 'contenttypes_tests.migrations',
+        'sessions': 'sessions_tests.migrations',
     }
     log_config = DEFAULT_LOGGING
     # Filter out non-error logging so we don't have to capture it in lots of