R1:7fbcdfc52ca8

Make CelerityController extend PhabricatorController

Summary:
Currently, CelerityController extends AphrontController, not PhabricatorController. (I think I imagined Celerity being somewhat stand-alone and didn't want to create a dependency.)

This creates a concrete problem if a static resource is missing, since we throw an exception, but the higher-level exception handlers depend on the User existing in order to show an appropriate response page. This is the only controller which doesn't extend…