Rename group name to "Guests"

[hurradieweltgehtunter]

Steps to reproduce

1. Go to user adminsitration panel
2. Open Group selection box on existing or new user

The group name "guest_app" seems to be very technical. I think it would be more comprehensive and nicer to rename the visible group name to "Guests"

[phil-davis]

User names have a displayName attribute. Groups do not have this attribute. But IMO it would be easy enough to add displayName for a group, and wherever group names are displayed to real people, show the group display name instead of, or in addition to the "internal" group name. The internal guests_app group name can stay the same, keeping backward-compatibility with all code that knows that name.