Fixed a bug in Image Macros where the image URL was wrong

Summary: Image URLs were accidentally inaccurate due to the macro ID being overwritten by the file ID when searching for users

Test Plan: Visited the macro page and searched for my own macros. They are now clickable.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3193

Differential Revision: https://secure.phabricator.com/D5936