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