Allow form to stay modal after uploading file

Use HTML5 FormData browser API together with FileList API to make
possible AJAX file uploads (see http://tinyurl.com/kmznw2n on FormData
examples). Feature detection is the same as in jQuery Form Plugin (see
http://tinyurl.com/n6v5dsx). Browsers without FormData support (most
importantly, IE<10) do not support setting custom AJAX request
headers, so form with File Upload field will lose modality in them
anyway.

Change-Id: Id3b91a49a71c6de314c6a7cbee5357fe13679490
Closes-Bug: #1158501
Merge "Allow form to stay modal after uploading file"