Introduce a quoted reply block rule for Remarkup

Summary:
Ref T4119. The tricky part here is that it's desirable to preserve other block-level formatting elements (like lists, other quotes, etc) inside the block.

To permit that, allow block-level elements to reparse their contents into more block-level elements, and essentially build a parse tree.

The new ReplyBlockRule strips off the ">>!" header and the ">" prefixes, then renders the rest normally, then slaps a blockquote around everything.

I haven't tweaked the styles too much, @chad might have thoughts on that. There should be enough tags/classes to get things rendering pretty flexibly.

I'm probably going to make the normal quote style work like this too (so block-level elements inside it are preserved). A big thing this will help with is inline replies into these blocks. This might imply keeping both quote styles relatively similar.

The selection of ">>!" is arbitrary, but I want some unique marker to indicate the meta/header text. This seemed as good as any.

Test Plan: {F149968}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: qgil, chad, epriestley

Maniphest Tasks: T4119

Differential Revision: https://secure.phabricator.com/D8953