[persistence] Make sure client receives UUID of newly generated combination group.
[persistence] Make sure client receives UUID of newly generated
combination group.

Do this to prepare for implementing `removeGroup` controller action.
Without a valid ID, it won't be possible to trigger the `removeGroup`
action for the correct resource.

itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE,itsjeyd/ODE