Fix Phriction rendering for initial install and 404 pages

Summary:
Depends on D19682. Ref T13202. We currently fatal when trying to render a timeline if:

• an install is fresh, so there are no pages yet, and you look at "/w/"; or
• you're looking at a Phriction page which doesn't exist (yet) like "/w/aadsflknadsflnf/".

Rendering a timeline and comment area doesn't make sense in these cases, so don't render them.

Test Plan: Hit both cases described above, got "new/empty page" prompts instead of fatals.

Reviewers: amckinley

Maniphest Tasks: T13202

Differential Revision: https://secure.phabricator.com/D19683