Implement Django import/export

[albinantti]

Although the import function maybe isn't the most necessary, being able to export orders into an csv file would be useful not only for the long term where someone might want to remove old orders from the system (either through a table drop from sysadmin or someone wanting to remove old menu items) or simply for the sake of documentation.

I've been told by Daniel that the library Django Import/Export is simple to implement - so it might be worth looking into.