Add a CLI workflow for testing that notifications are being delivered

Summary: Depends on D19865. Ref T13222. See PHI996. Provide a bin/aphlict notify --user ... --message ... workflow for sending test notifications from the CLI.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19866