Projects - add mail to project updates

Summary:
...which lets all the fancy settings for Email | Notify | Off be possible. Fixes T8164. Wasn't too sure the best way to break things up but members vs watchers felt meaningful to break out to me.

Also fixes a small bug where we were generating bad slug updated stories by messing with the signature of the slug data. Perhaps this fix isn't even good enough (the array_keys()) call and instead we'll need to implement transaction has effect and do a sort?

Test Plan: used ./bin/mail list-outbound and ./bin/mail show-outbound --id XX to verify reasonable emails were being generated. saw new preferences in settings.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8164

Differential Revision: https://secure.phabricator.com/D12868