/templates API endpoint has incomplete implementation
REST API has a read-only API for fetching the template of a mailing list. It should be noted that it is currently incomplete since it doesn't instantiate the content_type
correctly leading to 404 errors. This is useful to have for P and other web interfaces to show what the "current" template is before the users can override it.
The solution for this would be for someone to pass the accept https://falcon.readthedocs.io/en/stable/api/request_and_response.html#falcon.Request.accept header which can then be used by Core. We could also default to txt for the time being and not worry about the extensions right now.