External link redirector

This intercepts clicks on external links and passes them through a redirect page to thwart referer tracking.

@russellholt @zeen @nilnilnil @malbin It's not the most robust design ever, but it's better than what we have.

The ajax call sets the URL in the session, then we perform the redirect in a new window or the current one depending on the link's classes.

The template performing the redirect sets window.opener explicitly to stop the new site messing with our original window.