Add assistance buttons to Remarkup text areas

Summary: Unblocker for D3547. Adds markup assist UI (buttons which generate remarkup for you -- not WYSIWYG) to Remarkup text areas.

Test Plan: See screenshot. Clicked the buttons a bunch with selected/unselcted text. Results seem broadly reasonable.

Reviewers: btrahan, vrana, teisenbe

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T337

Differential Revision: https://secure.phabricator.com/D3594