Make blockquotes use parent/child rules
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.
Reviewers: chad, btrahan
Reviewed By: btrahan
Maniphest Tasks: T4119
Differential Revision: https://secure.phabricator.com/D8954