More flexible file uploading in the web interface

[rdmark]

I think it'd be helpful to be able to upload to the AFP share, or to an ISO, in addition to the image dir. In the upload_file() method, we currently ask the server for the currently configured default image dir and pass that to the Dropzone library. To make this dynamic, we would have to devise a way to pass an arbitrary value from the web form to Dropzone.