Add functionality to export to Microsoft Excel (8)

CSV's that are exported from Quizzard use the comma-separated standard, however, Microsoft Excel might expect the file separated with some other symbol depending on the regional settings, for example, a semicolon in the Dutch regional settings. This breaks the purpose of our exported CSV's in many cases, and there is no simple way to work around this.

This issue will be fixed by also providing the possibility to export to Microsoft Office Excel files (the xlsx format). This will be achieved by using the Apache POI library (https://poi.apache.org/components/spreadsheet/how-to.html).

To do:

• Create document interfaces that meet Quizzard's export requirements and are abstract enough to work with both CSV files and XLSX files (3).
• Implement those interfaces for CSV files (1).
• Implement those interfaces for XLSX files (1).
• Rename the CsvService to DocumentExportService and let it use the more abstract API (1).
• Edit different controllers to understand the way of exporting new documents (1).
• Update the links in the front-end for the different file formats (1).