R1:228496cdbe16

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

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