Redirect to group_owner/edit. See #11911

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/11907

To test, log in as a group owner and go to: User Settings -> My groups (tab) -> Edit a group
Edit the url to replace 'edit' with 'save'. E.g. webadmin/group_owner/save/4/ and hit enter.
You should be redirected back to the 'edit' page (previously got exception - see ticket).