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