Improve image macros

Summary:
Couple of small improvements:

• Delete randomon macro.
• Make name unique (deleting current conflicts randomly).
• Image macro must be alone on the line.
• Filter by name.

Test Plan:
Run SQL.
/file/macro/
/file/macro/?name=imagemacro
Try to create conflicting name.
Write this comment:

Test imagemacro.
imagemacro

Reviewers: aran, epriestley

Reviewed By: epriestley

CC: epriestley, Koolvin

Differential Revision: https://secure.phabricator.com/D2230