A few people have requested that groups could exist without a
mailing list to send to, and have it so relevant review requests
and reviews get e-mailed to any individual subscribers instead
of to the list.

This change (hopefully) does that.  It should be a fairly
self-explanatory patch.

Fixed up unit tests for new API and ran them.  The actual code
path for groups without mailing lists hasn't been tested, since
I'm not sure how to modify the fixture to add relevant test data.