Fix handling of notifications with project members

Summary: Fixes T7377. We don't expand projects into members when sending notifications right now. Instead, expand them.

Test Plan:

• Added a project as a reviewer to a revision, made a comment, saw project members receive a read notification + email (with appropriate preferences).
• There's meaningful test coverage on the core mail stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7377

Differential Revision: https://secure.phabricator.com/D12142