Add basic support for Herald outbound rules

Summary: Ref T5791. This is still very basic (no global actions, no support for matching headers/bodies/recipients/etc) but gets the core in.

Test Plan:
{F715209}

{F715211}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5791

Differential Revision: https://secure.phabricator.com/D13897