5261: When redirecting during an AJAX request, send the target URI in

5261: When redirecting during an AJAX request, send the target URI in
a JSON object {"href":"..."} instead of responding 302.

This lets us use "redirect_to X" to mean "send the user to page X"
regardless of whether the request is an XHR. Without it, client-side
code never sees the 302 at all: the browser handles the redirect
transparently, and the client-side code typically ends up trying to
parse HTML content as JSON.