Changes

[Pustalorc]

Removed orderby(x => x.Priority) as it should be handled by the permissions provider directly.
Changed firstordefault from x.Prefix != null to !string.IsNullOrEmpty(x.Prefix) so that if the prefix is empty, a prefix/suffix is still applied if any exists elsewhere.

[CLAassistant]

All committers have signed the CLA.

[aviadmini]

Checking prefix change makes sense
I'm not sure about the ordering, but it's probably better when this gives permission provider the control over groups