Make the "Reply" and "Quote" Remarkup rules more similar
The difference is that a reply starts with >>! In X, Y wrote:, although the attribution line is freeform text.
Make them both subclasses of a quoted-thing rule and push as much logic as possible up into the base class.
Test Plan: Added a unit test, made it pass. Existing tests all still pass, except for minor adjustments which look like improvements to me.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D20393