Add author field to image macro display

Summary:
this is a request from Facebook:

someone has added macros for common words like 'push', 'lgtm'.
We want to let he image macro page at least attribute to an owner so
that we can publicly shame whoever added the 800px 'clowntown' macro.

Test Plan:
test macros with/without author (all macro should have
author. This is just to be safe).

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: hwang, aran, arice, jungejason, epriestley

Differential Revision: 984