* Support downloading templates by URI, including mailman:// URIs.  This is
   used in welcome and goodbye messages, and supports both language and
   mailing list specifications.  E.g. mailman:///test@example.com/it/welc.txt

 * Schema changes:
   - welcome_msg -> welcome_message_uri
   - goodbye_msg -> goodbye_message_uri
   - send_welcome_msg -> send_welcome_message
   - send_goodbye_msg -> send_goodbye_message

 * New `ITemplateLoader` utility.