Renaming a table should also rename unique constraints

[pauloswald@…]

Once a project has built up a South migration history a table rename can be confusing for developers. You may get a django error page that says:

Exception Type: IntegrityError?
Exception Value: duplicate key value violates unique constraint "oldtablename_userprofile_user_id_key"

This will be hard to debug because the oldtablename may not be in the current schema.

The implementation of rename_table doesn't appear to drop/recreate constraints with the new table name.