Use authoritative user identity for revision author

Summary: We added a proper way to get the authoritative user a while ago, this
method just never got switched to use it.
Test Plan: Created a revision locally, was recognized as the revision author.
Reviewed By: jungejason
Reviewers: gc3, jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 566