Tighten up Phragment routes to prevent page crashes

Summary: Fixes T5418. These routes were a little more permissive than they should have been.

Test Plan: Hit those URLs without a path, got a 404 instead.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin, hach-que

Maniphest Tasks: T5418

Differential Revision: https://secure.phabricator.com/D9635