Move PhrictionContent from RemarkupInterface (deprecated) to PHUIRemarkupView

Summary:
Depends on D19092. Ref T13077. This modernizes markup rendering for PhrictionContent.

This is a little messy because table of contents generation isn't straightforward.

Test Plan: Viewed Phriction documents with and without 3+ headers, saw ToC vs no ToC. Edited/previewed documents. Grepped for affected symbols. Checked DarkConsole for sensible cache behavior.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19093