Make sure user group is loaded to update links

The resource_name of the groups controller was user_groups, causing
problems automatically loading the resource through RoledContoller.
This makes the resource loading explict and adds a test to make sure
the the update_links routes behaves correctly.

Closes #323
Merge pull request #328 from edpaget/fix-user-group-links

Make sure user group is loaded to update links