Rough implementation of the "Add Meme" button
Summary: This needs some tweaks but I'll follow up with @DeedyDas in T2353.
Test Plan: So many memes.
Reviewers: chad, btrahan, DeedyDas
Reviewed By: chad
CC: aran
Maniphest Tasks: T2353
Differential Revision: https://secure.phabricator.com/D4616