Diffusion - make projects work properly with commits

Summary: Fixes T3189. Now if you say #projects in a commit message they will associate nicely with the commit. Also we record transactions about all this project editing fun.

Test Plan: tested migration by associating some projects with commits and verifying they still showed up post migration. tested adding / removing projects by doing so from the UI, noting transactions written nicely as well

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Projects: #projects

Maniphest Tasks: T3189

Differential Revision: https://secure.phabricator.com/D10877