Modify Celerity to emit required resources on Ajax responses

Summary: Companion diff to D4003.

Test Plan: Rigged up a response which loaded a CSS file with body { background: red; }, verified page turned red after I clicked the button.

Reviewers: vrana, chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2086

Differential Revision: https://secure.phabricator.com/D4004