Move PhrictionContent from RemarkupInterface (deprecated) to PHUIRemarkupView
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