Detect 3rd party cookies and show an error screen

Sets a cookie during the `/provision` request.

Tests for cookie during the `/signin` request. If not present, displays this screen:
![](http://dl.dropbox.com/u/10060532/Screenshots/NUws.png)

Link goes to https://support.mozilla.org/en-US/kb/enable-and-disable-cookies-website-preferences?esab=a&s=3rd+party+cookies&r=2&as=s