* Add a `mailman send-digests` subcommand which replaces the functionality of
  the MM2.1 senddigests.py cronjob.

* Use mlist.data_path where appropriate instead of crafting it from
  config.LIST_DATA_DIR.  This makes it more consistent to switch to using the
  list-id as the data subdirectory.

* Refactor the to_digest handler so that we can implement
  maybe_send_digest_now() for the internal API.

* Fix some typos in subcommand --help summaries.