Allow users to receive email about pushes via Herald

Summary:
Fixes T4677. Implements a "send an email" pre-receive action, which sends push summaries.

For use cases where features are often pushed as a large number of commits (e.g., checkpoint commits are retained), using commit emails means users get a ton of email. Instead, this allows you to get an email about a push, which summarizes what changed.

Overall, this is basically the same as commit email, but more suitable for some workflows.

Test Plan:
Wrote some rules, then made a bunch of pushes. Got email like this:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4677

Differential Revision: https://secure.phabricator.com/D8618