Add Link button to Remarkup assist

Summary: Believe it or not, I forgot how to create a link in Remarkup.

Test Plan: Clicked on it with selected URL, selected text and without a selection.

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: epriestley, aran, chad

Differential Revision: https://secure.phabricator.com/D7336