Fix "edit" for Maniphest comments

Summary:
Ref T2217. Fixes T3876. We incorrectly have a unique key on (authorPHID, transactionPHID), which prevents saving multiple versions of a comment.

I'm not entirely sure why this exists. I think it came from Pholio (where it works for inlines, because it has an additional component, but maybe should be adjusted) but we might need to wipe it out of more apps too.

Test Plan: Edited a comment in Maniphest.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217, T3876

Differential Revision: https://secure.phabricator.com/D7102