Add author to differential.getrevisionfeedback
Summary: Add the author PHID to the differential.getrevisionfeedback conduit api
method
Test Plan: issue differential.getrevisionfeedback query via conduit against a
valid revision and verify author phid is included in results
Reviewers: epriestley
Reviewed By: epriestley
CC: epriestley, aran, jungejason, tuomaspelkonen, jonathanhester
Differential Revision: 1190