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:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4119

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