Copy function for files/mainfiles

Would allow to use existing entries as a "template" #785 (closed).

• 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")