Handle CSRF errors for JSON requests

Currently, the handler for CSRF errors works for HTML requests and JS requests, but not for JSON requests:

This is resulting in a lot of errors from the hit count endpoint in New Relic. The handler should be modified to return a reasonable value for JSON requests. (In fact, since the JS handler returns JSON, it might work to just return the same thing for both JS and JSON requests.)

Testing Instructions:

1. Disable all cookies.

2. View a work on staging.

3. Make sure that the ActionController::UnknownFormat error doesn't appear in New Relic.