Add `group_backend` option to mirroring the circle as a group

[tortuetorche]

The main goal is to have better integrations with Nextcloud applications and clients.

The group name is automatically suffixed by Circle. And you can customize the prefix and/or suffix group name with the group_backend_name_prefix and group_backend_name_suffix options.

You should also hide these mirrored circles in Nextcloud applications and clients. Because their equivalent group will be used instead, via the allow_listed_circles option set to 0.

Fix https://github.com/nextcloud/circles/issues/363