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.