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: {F134929} Reviewers: btrahan Reviewed By: btrahan Subscribers: epriestley Maniphest Tasks: T4677 Differential Revision: https://secure.phabricator.com/D8618