#21 Export to excel

Further tabs and their content queries can be implemented under the XlsWriter and ExportRepository classes following the example presented for Gels. Since export won't be connected to the rest of the application logic I decided to give it its own repository for implementing the queries.

Since we expected custom queries and views to be used for the tables, I also went for the approach of not returning ORM models but headers and raw row data.

If you wish to rename the columns presented in excel, either add an alias to the select statement (for example Gel.transfer.label('Transfer Date')) or use translation strings.