Copy function for files/mainfiles
Would allow to use existing entries as a "template" #785.
existing "POST uploads/.../rawfile" should get an "existing_path" query parameter that causes the creation of a copy.
another parameter "move" could allow to move a file from "existing_path"
add GUI function to create the copy/move in the file browser. Should allow to enter a new name (potentially an upload "/root/path")