FIX: Don't redirect XHR/JSON requests when login is required (PR #15093)

When redirecting to login, we store a destination_url cookie, which the user is then redirected to after login. We never want the user to be redirected to a JSON URL. Instead, we should return a 403 in these situations.

This should also be much less confusing for API consumers - a 403 is a better representation than a 302.