Web editor/browser for image macros

Summary:
Restores the old "pokedex" feature and allows easy definition of new macros.

Only good can come of this!

Critical feature!!

Test Plan:
nyancat

Reviewed By: tuomaspelkonen
Reviewers: aran, bh, tomo, jungejason, tuomaspelkonen
CC: aran, tuomaspelkonen
Differential Revision: 220