Rewrite tracked external links

For links that point at our external link tracker, which are links that
look like:

    <a href="/g?url=URL" />

We should alter browser behaviour to preserve visited link and hover
behaviour. We do this by:

* Rewriting the link at DOMReady time to use the external target instead
  of our redirector
* Using jQuery's `click` event, which tracks keypresses and other kinds
  of actions as well as clicks, to put the original value back when
  following the link. Due to the way browsers behave this should rewrite
  the `href` and follow the internal link.

This should preserve all the usual browser behaviour of cmd+click, right
click etc.