Clean up riot-web UI (disable custom URLs and guest usage)

This playbook does not set up guest access in Synapse anyway,
so until the need comes (or someone asks for it), guest access
is removed from riot-web's UI too.

As for supporting custom URLs, this is also not something
that seems like it'd be useful to most deployments.

roles/matrix-server/templates/riot-web/config.json.j2

@@ -1,6 +1,8 @@
 {
     "default_hs_url": "https://{{ hostname_matrix }}",
     "default_is_url": "https://vector.im",
+    "disable_custom_urls": true,
+    "disable_guests": false,
     "brand": "Riot",
     "integrations_ui_url": "https://scalar.vector.im/",
     "integrations_rest_url": "https://scalar.vector.im/api",