Merge branch 'group-join' into 'nightly'

Add User_group::getObjectType

Since 174586bd512, I'm unable to join groups with the logs complaining
about getObjectType not existing on the User_group object.

Note: I'm not sure if this is the correct value to return here, but I
was able to joing groups successfully using this. Let me know if it needs
to be changed and I'll update the Merge Request!

See merge request !56