Action token requirement for link redirector is causing crashes.

Nix use of that and perform some basic referer checking. If there's a
failure, render an annoying interstitial page.

This is infinitely better than crashing.
Don't bother encoding the URL, since there's only one parameter now.
Merge pull request #306 from duckduckgo/external-links-no-action-token

Action token requirement for link redirector is causing crashes.