Resolves "Redundant duplication of Multi-linked contents during CW copy"

This MR fixes #336 (closed) by checking if the copy of the content exists and making sure it is up-to-date.

How it works

• copyFileById now adds an extra param called "copied_from" in File's metadata field and checks the existing files against this param to make sure there is only one copy of a file during CW copy.
• copyFolderById like above, adds an extra param called "copied_from" in Folders's data_content field and later checks the exiting folders against this params, also with the help of compareFoldersContents method, it checks if both source and existing folder have the same content and type and so on.
• In case more checkers are required to compare the folder contents, we can add them in compareFoldersContents.