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

Also addresses issue #1439.

I don’t like the binding iframe.on('load') events in the onclick handler, but apparently Entwine doesn't support binding on iframes.

AssetAdmin and HtmlEditorField support added
Merge pull request #2719 from kinglozzer/uploadfield-deferred-loading

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