Include OpenGraph prefix material in <html> tag if OpenGraph is enabled
Summary: Ref T13018. Discourse doesn't seem to be picking this up yet (see https://discourse.phabricator-community.org/t/opengraph-test-topic/762) so maybe it really needs this meta-meta-XML stuff?
Test Plan: Will push.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13018
Differential Revision: https://secure.phabricator.com/D18781
