File conflicts dialog fixes

currently when you upload a file and then try to delete it you will get an exception. The reason is that a cancelUpload is tried which contains dead and broken code. Canceling individuals is currently not supported because we have no placeholders for individual uploads. The files are added after they have been uploaded.

This PR removes the old code cleans up several minor issues. see commits

@karlitschek @kabum @DeepDiver1975