Warn about matrix_user_uid/matrix_user_gid

We don't really need to fail in such a spectactular way,
but it's probably good to do. It will only happen for people
who are defining their own user/group id, which is rare.

It seems like a good idea to tell them that this doesn't work
as they expect anymore and to ask them to remove these variables,
which otherwise give them a fake sense of hope.

Related to #486 (Github Pull Request).

roles/matrix-base/tasks/main.yml

@@ -2,6 +2,11 @@
   tags:
     - always
 
+- import_tasks: "{{ role_path }}/tasks/validate_config.yml"
+  when: "run_setup|bool"
+  tags:
+    - setup-all
+
 - import_tasks: "{{ role_path }}/tasks/clean_up_old_files.yml"
   when: run_setup|bool
   tags:

roles/matrix-base/tasks/validate_config.yml

@@ -0,0 +1,11 @@
+---
+
+- name: (Deprecation) Warn about unused user/group variables
+  fail:
+    msg: >
+      The `{{ item }}` variable defined in your configuration is not used by this playbook anymore.
+      User/group creation is now dynamic. You can remove these variables from your configuration, as they have no effect on anything.
+  when: "item in vars"
+  with_items:
+    - 'matrix_user_uid'
+    - 'matrix_user_uid'