Make it less confusing to create root-level Phriction doc

Summary: Without an existing root document, Phriction shows a nice little "fake" document as the landing page, which has its own nice "Edit this document" button. When showing that page, don't also render the standard "New Document" breadcrumb in the top right. That button always prompts first for a slug name, which is silly when the root document doesn't exist (because the slug name is required to be '').

Test Plan: Loaded Phriction with and without a root document.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19863