Use a randomly-generated GUID for links instead of an auto-increment ID

The GUID is a random string representing a number in base 58 between 0
and 58^10, with an extra prefix for namespacing.
Add some framekiller mitigation

This does two things:

(1) For sites that try to hijack the page and redirect (like the NYT),
    this prevents it with an HTML5 attribute called "sandbox". This
    feature is not supported in older browsers, so adding hackier
    JavaScript framekiller-killer techniques remains a TODO.

(2) Fetches the submitted URL and checks if the X-Frame-Options header
    is set. If it is, the page is likely to not be displayable in an
    iframe, so we don't show the iframe.