Hand-check unique_together constraints with null values.
Django has a bug where unique_together constraints for foreign keys aren't

enforced properly when one of the foreign keys is null (well, technically it's

a database bug). This meant that users who weren't using local sites could

create Groups or Repositories that had conflicting names and paths.
This change implements the clean() method for the Group and Repository

models. This method is automatically called from ModelForms, which means the

admin site is fixed automatically.
The web API has a similar issue, but only for repositories. The Group

create/update methods had an implementation which already checked for these

sorts of problems. I haven't implemented a unit test for the repository API

changes because this change is currently on 1.7.x, but I'll add it to the to-do

list for master.

Tried to create a Group with a conflicting name from the admin UI.
Tried to create a Repository with a conflicting path from the admin UI.
Tried to create a Repository with a conflicting name from the admin UI.
Tried to create a Repository with a conflicting path from the web API.
Tried to create a Repository with a conflicting name from the web API.
Ran unit tests.