show system groups in contacts app (if system address book is enabled)

Steps to reproduce

1. create users and groups in user management
2. go to contacts app, make sure system address book is enabled and all other address books disabled
3. you see the users, but not the (system) groups they belong to

Expected behavior

System groups should be shown in the left sidebar, like regular cardDAV-contact groups in other address books.

Users interact with groups (=groups in user management) a lot, for example when creating Talk conversations or when sharing files/folders. The user management is the only place where you can see who is in which group - but most people are users, not admins, so they cannot see this. Users want an overview of what groups exist and who is in them.

Note that no information would be "exposed", if I am allowed to add user groups to a Talk conversation, this way I can find out who is in that group anyway.

Of course this should respect the settings made for sharing on the server,

Actual behavior

no user groups shown

Contact version

5.5.2

Reproduce the issue in the Contacts app with the system address book enabled and other address books disabled, following the listed user and group setup. Trace how the left sidebar loads groups and how server sharing settings constrain visibility. Done means system user groups appear with their membership visibility respected, alongside regular CardDAV contact groups.