Convert Remarkup to safe HTML

Test Plan: None.

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4919
Convert everything to safe HTML

Summary: Sgrepped for `"=~/</"` and manually changed every HTML.

Test Plan: This doesn't work yet but it is hopefully one of the last diffs before Phabricator will be undoubtedly HTML safe.

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4927