Display full picture in image macro editing

Summary:
Users are complaining that they don't see how the image macro looks until they use it.
Click leads to edit form.
Display it there.

Test Plan:
Edited macro.
Attempted to create macro with duplicate name.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3636