Fix Dashboard to-group for unjoined public groups
Addresses issue

https://code.google.com/p/reviewboard/issues/detail?id=3204

reviewboard/manage.py test -- reviewboard.reviews.tests:ViewTests.test_dashboard_to_group_with_unjoined_public_group  reviewboard.reviews.tests:ViewTests.test_dashboard_to_group_with_unjoined_private_group

Running dependency checks (set DEBUG=False to turn this off)...

Warning: p4python (>=07.3) not found.  Perforce integration will not work.

Warning: hg not found.  Mercurial integration will not work.

Warning: bzrlib not found.  Bazaar integration will not work.

Warning: PyLucene (with JCC) is required to enable search. See the documentation for instructions.

Warning: mtn binary not found.  Monotone integration will not work.
Please see http://www.reviewboard.org/docs/manual/dev/admin/

for help setting up Review Board.

Creating test database for alias 'default'...

Testing dashboard view with to-group and unjoined public group ... ok

Testing dashboard view with to-group and unjoined private group ... ok

Ran 2 tests in 1.462s
OK

Destroying test database for alias 'default'...