Add matrix_group_system to configure whether the matrix group is also created as a system group

We create the `matrix` user as a system user (smaller uid), so it makes
sense to do the same for the `matrix` group.

roles/custom/matrix-base/tasks/setup_matrix_user.yml

@@ -10,6 +10,7 @@
     name: "{{ matrix_user_groupname }}"
     gid: "{{ omit if matrix_user_gid is none else matrix_user_gid }}"
     state: present
+    system: "{{ matrix_group_system }}"
   register: matrix_group
 
 - name: Ensure Matrix user is created