Give Diviner a non-legacy root controller

Summary: Ref T988. Instead of hard-coding the application landing page, make the Diviner root show books if any have been generated. Otherwise, show a helpful message about how to generate documentation locally.

Test Plan:
{F122723}

{F122724}

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D8416