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/tasks/setup_uninstall.yml

@@ -19,7 +19,7 @@
         path: "{{ devture_systemd_docker_base_systemd_path }}/matrix-cactus-comments.service"
         state: absent
 
-    - name: Ensure Matrix cactus comments paths don't exist
+    - name: Ensure matrix-cactus-comments paths don't exist
       ansible.builtin.file:
         path: "{{ matrix_cactus_comments_base_path }}"
         state: absent