Serve celerity resources from multiple maps

Summary: Ref T4222. Adds the map name to Celerity resource URIs, so we can serve out of any map.

Test Plan: Poked around, verified URIs have "/phabricator/" in them now.

Reviewers: btrahan, hach-que

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4222

Differential Revision: https://secure.phabricator.com/D7877