Make repository names and paths unique per local site.
    
It's possible that people in different local sites will create the same
repository name, or even the same repository path. Since a local site acts like
its very own reviewboard server as far as the webapi and ui are concerned, these
fields should be unique_together with local_site instead of globally unique.

Created a "duplicate" repository with a different local site as another.
Tried to create a "duplicate" repository with the same local site as another and
saw the expected errors on the name and path fields.
Tested evolution.