Add matrix-cactus-comments-client role

This is split out from matrix-cactus-comments (see 241779b583),
but also heavily inspired by `matrix-static-files`.

roles/custom/matrix-cactus-comments-client/tasks/validate_config.yml

@@ -0,0 +1,11 @@
+---
+
+- name: Fail if required matrix-cactus-comments-client settings not defined
+  ansible.builtin.fail:
+    msg: >-
+      You need to define a required configuration setting (`{{ item }}`).
+  when: "vars[item] == ''"
+  with_items:
+    - matrix_cactus_comments_client_hostname
+    - matrix_cactus_comments_client_path_prefix
+    - matrix_cactus_comments_client_container_network