sakaiproject/Hilary — pull request #528 by simong (2013-06-04 16:33:10+00:00)
Title: Detect links that don't allow iframe embedding

As part of the preview processor for links, we can do a HEAD request to the link and check the `X-Frame-` header to see if the link allows iframe embedding. If it doesn't allow it, a property can be set on the previews object, so the UI doesn't attempt to show the non-embeddable URL.