R1:91eb22cb3a8b

Migrate Project slugs to modular transactions

Test Plan: Unit tests all pass. Added/removed/altered some project hashtags and observed expected transactions in timeline.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17952