Automatically send (not-so-great) email notifications for upcoming events

Summary: Ref T7931. This is still quite rough, but should technically send vaguely-useful email as part of the standard trigger infrastructure.

Test Plan: Ran bin/phd start, created an event shortly, saw reminder email send in bin/mail list-outbound.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7931

Differential Revision: https://secure.phabricator.com/D16784