Fix Dashboard to-group for unjoined public groups

This change updates the dashboard to use Group.is_accessible_by instead of
limiting things to users' joined groups. This allows users to use the dashboard
to see watched groups as well as joined groups.

Testing Done: Ran unit tests.

Bugs closed: 3204

Reviewed at https://reviews.reviewboard.org/r/5396/
Merge branch 'release-1.7.x'
Merge remote-tracking branch 'origin/master'