Let Mingle card references point to Thoughtworks' instance

The wmf hosted Mingle instance became read-only and teams now use the
Thoughworks instance. New cards are only available there, and hence we
adjust the link target.

Old cards from wmf hoste Mingle instance are also available in the new
Thoughtworks instance, so there is no need for special treatment of
old cards.

Change-Id: I0c733fb2fc226631396454b4c6b4a5f1eb4f1b29