Fix 3rd party cookie issues with Safari

By default, Safari blocks 3rd party cookies. That means that embedded pads won't render as their authorization cookies can't be set.

Hackpad has a redirect URL we can use to set a cookie before a pad is loaded. Since there are situations where a Hackpad may be rendered as the result of a POST request, we can't just redirect before loading the pad. However, we can set the cookie on login so any pads will load properly.

https://hackpad.com/?setCookie=1&contUrl= is the URL to use.