Enable image macros on the wiki

Summary: D2230 did the heavy lifting spoken of in the comment this diff deletes. Also remove 'macro' => false.

Test Plan: Added an image macro to a wiki document and it showed up

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1023

Differential Revision: https://secure.phabricator.com/D2668