R1:bfea830d09fd

Add email preferences to receive fewer less-important notifications

Summary:
A few similar requests have come in across several tools and use cases that I
think this does a reasonable job of resolving.

We currently send one email for each update an object receives, but these aren't
always appreciated:

- Asana does post-commit review via Differential, so the "committed" mails are
useless.
- Quora wants to make project category edits to bugs without spamming people
attached to them.
- Some users in…