Migrate Project parent and milestone to modular transactions

Test Plan: Unit tests pass. Went through the UI for creating new subprojects and milestones, but didn't setup some API calls to check that all the validation errors were still caught.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D17999