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/defaults/main.yml

@@ -177,6 +177,7 @@ matrix_container_global_registry_prefix_override: ""
 
 matrix_user_username: "matrix"
 matrix_user_groupname: "matrix"
+matrix_user_system: true
 
 # By default, the playbook creates the user (`matrix_user_username`)
 # and group (`matrix_user_groupname`) with a random ID.