Fix Twitter cards / post image retrieval

Before there was a twig filter for plunging into the content of a post.
Now that images are set manually though, use this with a filter to
ensure a URL has a scheme on it
Fix empty image issue
Merge branch 'twittercards'
Propagate a couple of live hotfixes
Allow uploading generic files via media uploader

So I can now upload JS and CSS files etc.