Move /bin/navdump Handlers methods to the models they are for

[hmpf]

By moving the staticmethods on Handlers to the models (or managers) they convert, they can be tested together with that model, partially solving #1867 for /bin/navdump.

Also, when the definition of the export format is kept in the same file as the model, propagating changes from the model to the format is easier.

I suggest:

1. Header as attribute on model
2. Model.export_csv() for a single model.
3. queryset.export_csv() for a queryset().

Maybe standalone functions being reused by 2 and 3, export_csv_<model>, or maybe drop 2 and only do 3.

[hmpf]

For the queryset-methods: Return a generator of byte-strings encoded as utf-8, so that encoding of console/terminal doesn't matter.