'Create Document' pre-fills with current slug

Summary:
Fixes T2667

The 'Create Document' link up in the application crumbs now prefills with the slug of the current document - if available, that is

Test Plan: verified that nothing crashed, that the correct slug was pre-filled, and that pages could be created

Reviewers: alexshtuk, epriestley

CC: aran, Korvin

Maniphest Tasks: T2667

Differential Revision: https://secure.phabricator.com/D5273