Store authors on image macros

Summary:
Currently, the author of an image macro is read from the attached file. This is messy and necessitates a join, and is not always correct. Instead, store the data when the macro is created.

This lays the groundwork for generalizing ApplicationSearch here. Ref T2625.

Test Plan: Migrated existing macros, created a new macro, checked web UI.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6071