Phriction shows document hierarchy above page content

Summary: Added phriction config flag to show document hierarchy above page content.

Test Plan: After changing config option 'phriction.show-document-hierarchy-at-the-top' to true, document hierarchy will be shown above page content.

Reviewers: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6870