Add missing needReviewerAuthority() for differential.createcomment

Summary: For some actions (like accept) we need to load reviewer authority so we can figure out if the actor can act on behalf of project reviewers, etc.

Test Plan: Will make @dctrwatson do it.

Reviewers: dctrwatson, btrahan

Reviewed By: btrahan

Subscribers: chad, aran, dctrwatson, epriestley

Differential Revision: https://secure.phabricator.com/D8505