Convert Remarkup to safe HTML

Summary: This could be slow but we cache where it matters.

Test Plan:
We have quite good unit tests.
Didn't test this in Phabricator yet (neither in //master// nor //phutil_tag//).

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4918