Update community authentication to pass an arbitrary datablock instead of url

This makes it possible to pass URLs that will fail when they end up being double
escaped in some cases, since they contain non-url-safe characters. Instead, they'd
be base64-encoded, and thus safe.

Also update the django community auth provider to do just this, including encrypting
the data with the site secret key to make sure it can't be changed/injected by
tricking the user to go directly to the wrong URL.