Special case the "added projects" transaction in mail when creating objects

Summary: Fixes T10493. See that task and inline comments for discussion.

Test Plan:
Created an object with some projects, saw the transaction in resulting mail:

{F1600496}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10493

Differential Revision: https://secure.phabricator.com/D15942