Undo sessions being driven by UUID.

Basing sessions on UUIDs must be done in phases. First all users
must obtain an UUID. Once a backfill has given all previous users
UUIDs and new users are being generated with UUIDs, then we can
actually change the session to be based on that value.

test/test_api_security.py

@@ -79,7 +79,7 @@ class ApiTestCase(unittest.TestCase):
       with client.session_transaction() as sess:
         if auth_username:
           loaded = model.get_user(auth_username)
-          sess['user_id'] = loaded.uuid
+          sess['user_id'] = loaded.id
           sess['login_time'] = datetime.datetime.now()
         sess[CSRF_TOKEN_KEY] = CSRF_TOKEN