FIX: Defer loading of UploadField EditForm iframes (fixes #2711)

NOT TESTED IN IE YET.

Also addresses issue #1439.

I don’t like the binding iframe.on('load') events in the onclick handler, so might rewrite that.

CSS fixes for ye olde IE
Added index docs to DataObject

Thanks to @oddnoc for getting this started, see
https://github.com/silverstripe/silverstripe-framework/pull/2686