Remove deprecated functionality and maintain code context of new links

This pull request removes support for old versions of the extension that changed the MIME type of injected applications to `iframe` so the remote server could send an injectable version of the application. Since no extensions currently use this functionality, it has been removed.

I also fixed an issue where the hosted version of the new.html applications would show the user the local-code version of the URL when they clicked the link. It still worked, but it was very ugly.