Update doc loader for compat with old viewers
Update doc loader for compat with old viewers

* Fall back to old viewer’s `DV.load` if new viewer’s `DV.immediatelyLoadDocument` isn’t available
* Collapse `window.DV` to `DV` where possible
* Replace `querySelector` with `document.getElementsByTagName` for CSS/JS insertion sites

#420

documentcloud/documentcloud,documentcloud/documentcloud,documentcloud/documentcloud,documentcloud/documentcloud