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