Add matrix_user_system variable for controlling if matrix user is created as a system user

This defaults to `true`, as before.

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

@@ -20,7 +20,7 @@
     group: "{{ matrix_user_groupname }}"
     home: "{{ matrix_base_data_path }}"
     create_home: false
-    system: true
+    system: "{{ matrix_user_system }}"
   register: matrix_user
 
 - name: Initialize matrix_user_uid and matrix_user_gid