Enable hovercard controller to send ajax ready responses

Summary: Refs T1048 - Resolves the TODO

Test Plan: Used in a future diff. Used JX.log to verify JSON format and correct gibberish HTML output of input (I will never understand how browsers can be so evil and ignore `Content-Type`).

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1048

Differential Revision: https://secure.phabricator.com/D5557