Don't show hint if network fails within 50 ms

Summary: I'm hopeful this will prevent cheating of the sort seen where students disable Wi-Fi on a device and then take a free hint.

Test Plan:
Locally (sans webapp), take hints normally. Load webapp via LAN (not loopback) and take hints normally. Turn off Wi-Fi and note that the hint button does nothing now. Also did the same in an IE10 VM with Wi-Fi enabled and disabled.

Typical requests on my Mac Chrome take 4-9 ms to time out (one took 14); on IE10 timings were in the 20-30 ms range. Hopefully 50 ms is high enough to catch them all but low enough to be unnoticeable. (Future idea: show the hint immediately but re-hide it if the network fails? Much more code complexity though.)

Perseus One preview and khan-exercises timeline work as before.

Reviewers: jack, eater

Reviewed By: eater

Subscribers: marcia, stephanie, mgp, ben

Differential Revision: http://phabricator.khanacademy.org/D10084