Redo the way matryx-rageshake configuration is done

This makes it consistent with the rest of the playbook:
- there's a default config which has various variables controlling
  settings
- there's also an `_extension_yaml` variable, which lets you override it

roles/custom/matrix-rageshake/templates/config.j2

@@ -1,2 +0,0 @@
-api_prefix: {{ matrix_rageshake_config_api_prefix | to_json }}
-{{ matrix_rageshake_config | to_nice_yaml(indent=2, width=999999) }}

roles/custom/matrix-rageshake/templates/config.yml.j2

@@ -0,0 +1,4 @@
+# Default configuration for Rageshake.
+# To extend it, use `matrix_rageshake_configuration_extension_yaml`.
+
+api_prefix: {{ matrix_rageshake_config_api_prefix | to_json }}