R1:1aad40b7bf02

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…