File - add transactions and editor

Summary: this ends up being a little weird since you can't actually edit files. Also, since we create files all sorts of ways, sometimes without even having a user, we don't  bother logging transactions for those events. Fixes T3651. Turns out this work is important for T3612, which is a priority of mine to help get Pholio out the door.

Test Plan: left a comment on a file. it worked! use bin/mail to verify mail content looked correct.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran, wez

Maniphest Tasks: T3651, T3612

Differential Revision: https://secure.phabricator.com/D6789