Allow PublicBody CSV import fields to be configurable

This PR makes the list of fields which can exist in the PublicBody batch
upload CSV configurable via the confi file, rather than being hard-coded into
the model.

I'm not sure if this is something that would be wanted or not, but it solves
our particular use-case in Panama quite neatly (much more neatly than the
alternative of overriding things in the theme). Therefore, I've thrown this
together very quickly to see if the general concept is acceptable and if so,
elicit comments on the approach.