fixes upload and prevent doubleclick for #430

The issue https://github.com/NuGet/NuGetGallery/issues/430 is very old and apart from the original issue text the complete AsyncFileUploadManager had a major bug in this line.
var _isWebkitBrowser = $.browser.webkit; 

The $.browser was deprecaded in jQuery 1.9 and this line prevent the async file upload. The same error can be seen on NuGet.org.

In the PR I set the isWebkitBrowser to false but I think the code could be removed.

With this in place I disabled the submit button on "submit" to prevent doubleclicking and the async file upload works again.
Tested in Chrome and IE11.

As far as I know the iFrame Workaround is not needed with modern browsers. Let me know if I should delete the _isWebkitBrowser reference and the iFrame building code.