Support `:emoji:` in Remarkup

Summary: Ref T1191. This actually works without T1191, but makes emoji use on the desktop easier.

Test Plan: {F210416}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1191

Differential Revision: https://secure.phabricator.com/D10605