Initiated the work on #9239

The goal is to add an upload button for styles,
that will upload to our website, darktable.org.
Most of the gui is rendered, but it
doesn't do anything useful yet.
Creating and exposing before/after images
Successfully exporting/uploading styles + previews

There are still things to do, I've listed them in the
feature request here: http://darktable.org/redmine/issues/9239