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 general are very sensitive to email volumes, and this gives us

a good way to reduce the volumes without incurring the complexity of
delayed-send-batching.

The technical mechanism is basically:

• Mail may optionally have "mail tags", which indicate content in the mail

(e.g., "maniphest-priority, maniphest-cc, maniphest-comment" for a mail which
contains a priority change, a CC change, and a comment).

• If a mail has tags, remove any recipients who have opted out of all the

tags.

• Some tags can't be opted out of via the UI, so this ensures that important

email is still delivered (e.g., cc + assign + comment is always delivered
because you can't opt out of "assign" or "comment").

Test Plan:

• Disabled all mail tags in the web UI.
• Used test console to send myself mail with an opt-outable tag, it was

immediately dropped.

• Used test console to send myself mail with an opt-outable tag and a custom

tag, it was delivered.

• Made Differential updates affecting CCs with and without comments, got

appropriate delivery.

• Made Maniphest updates affecting project, priority and CCs with and without

comments, got appropriate delivery.

• Verified mail headers in all cases.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley, moskov

Maniphest Tasks: T616, T855

Differential Revision: https://secure.phabricator.com/D1635