Add piles of special casing to support "mailto:"

Summary: Ref T3949. Bleh.

Test Plan: Unit tests.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3949

Differential Revision: https://secure.phabricator.com/D8962
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
Make blockquotes use parent/child rules

Summary:
Ref T4119. This makes blockquotes generally work better (lists, code blocks, and other block elements will continue to work inside them). Particularly, it makes inline replies inside of quoted blocks tend to work better.

This changes some edge cases of the semantics slightly, but overall the results should be more reasonable and consistent.

Test Plan: {F149970}

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4119

Differential Revision: https://secure.phabricator.com/D8954