Add a worker to automatically GC expired app specific tokens

Fixes https://jira.coreos.com/browse/QUAY-822

data/model/test/test_appspecifictoken.py

@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timedelta
 from mock import patch
 
 import pytest
@@ -36,7 +36,7 @@ def test_gc(expiration, initialized_db):
   token = create_token(user, 'Some token', expiration=expiration_date)
 
   # GC tokens.
-  gc_expired_tokens(user)
+  gc_expired_tokens(timedelta(seconds=0))
 
   # Ensure the token was GCed if expired and not if it wasn't.
   assert (access_valid_token(token.token_code) is None) == is_expired