Move HTML-only intraline diff code to Phabricator

Summary:
Ref T13395. This code relies on PhutilHTML, but that's moving to Phabricator.

It has no callers in libphutil or Arcanist.

Test Plan: Looked for callers.

Maniphest Tasks: T13395

Differential Revision: https://secure.phabricator.com/D20978