Feature request: Link existing groups instead of creating new ones

[asta-tud-deploy]

Currently the App needs at least three groups to define shiftworkers, shiftadmins and skillgroups, to even configure it in the admins panel. At least it does not work for me.
Since we use an LDAP and a very complex rights-group management, adding unnecessary groups not fitting the naming schema is very unpleasent.
It would be much nicer to be able to link groups to the corresponding rights in the settings without having to set up hard coded groups, just to change them afterwards in the settings.

Thank you :)