Make Celerity a real application

Summary: Ref T5702. This primarily gets URI routing out of Aphront and into an Application, for consistency.

Test Plan: Loaded some pages, got static resources.

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5702

Differential Revision: https://secure.phabricator.com/D10696