External link redirector

This intercepts clicks on external links and passes them through a redirect page to thwart referer tracking.

@russellholt @zekiel (sorry @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.
Isn't this going to happen too late? mousedown will fire before click, and this doesn't bind the mousedown event until after click.

Also addEventListener won't work in old IE, better to just use jQuery, ```$(this).on('mousedown', fn)```

How about this?
```
$("a[href^='http']".each(function(el) {
  if this.href.indexOf(location.hostname === -1) {
    $(el).on('mousedown', set_url_to_redirect(this));
  }
});