Add view source groups [PD-719]

The new test tests the base functionality such that a model with zero functionality can be tested. As PD-719 is just DB support, there's not much else to test.

@haliscooper This may not look exactly like the SQL you provided, but it should be adequate. Through tables are created automatically when you specify a ManyToManyField on a model. It has three columns, one for the pk of each side of the relationship and one for its own pk.

I used a boolean rather than bit field as Django doesn't have a built-in representation for them and I didn't feel that adding a library for one field would be valuable.

All tests pass.