CW: Add terms of use to export

This MR fixes #1738 (closed), fixes #921 (closed),

Description

By exporting files in a block (e.g. DownloadBlock) with or without License (terms-of-use), the exported files won't get any term-of-use relationships data. However on the other hand upon importing the data, it looks for that relationship attribute and set that if exists. Therefore, the problem occures becasue of the absence of this relationship.

Solution

• Upon exporting fileref data, it checks if that terms-of-use attribute exists in the relationships, and then adds it to the export info (file.json)
• Upon importing and in the createFile method, the checker was faulty, as it assumed that the terms-of-use always exists! So, the solution was to make that checker more accurate there!