Prevent circles of collectives from being deleted in Contacts

Right now, it's easy to accidently destroy a collective by deleting its underlying circle from the Contacts app.

Fortunately, Circles recently added a new feature to flag a circle as managed by an app which disallows it to remove the circle from the Contacts app. We should set this flag for circles of collectives.

Once new Nextcloud versions with this feature have been released, we should probably also add a migration step to set this flag for all circles of existing collectives.