Make the "Reply" and "Quote" Remarkup rules more similar

Summary:
Previously, see D20327. The "Reply" and "Quote" remarkup rules currently have some copy/pastey-feeling code, and the "Reply" rule wasn't touched by D20327.

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.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20393