Collabdocs predating 8.0.0 preview images are blank

The following happens when processing a collabdoc:
* Get the content item's latest revision
* Get the document's html fragment from the latest revision.
    - Pre 8.0.0 this used to be a fragment (so no `<html><body>`)
    - In 8.0.0 we upgraded to the latest etherpad version where this html fragment is now a full blown html page
* Get the body of the html page, wrap it in our own html structure and write it to a temp file
* Process it through webshot

Now, all documents who haven't been touched since we upgraded etherpad will still have the old html fragment on their latest revision.

We probably want to put some logic in place that checks both of them and parses out the correct HTML fragment. We could do a point release, reprocess all collabdocs and then take out the code again.
Do we need to have the Etherpad documents wrapped in HTML? All we do with them during preview processing is unwrap them again and wrap them into something else.