Ignore emails matching certain regexes.
Extract original sender from forwarded emails.

While the controller will listen for new requests at
invite@<controller-id>.appspotmail.com, forwarded emails should go
to forward@<controller-id>.appspotmail.com, so we know to treat
them differently.

This works for single e-mails forwarded from one's personal inbox.
Unfortunately, Google Groups strips the original sender's address
if you forward from the mailing list interface.  Also, digest mode
is not supported yet.