Export and Import Attendees

Adds two functions to export and import all attendees in the database. Will be super-useful for data entry as well as integration with a CSV.

Bonus: Works dynamically - unless we add a new type of column, this requires no management.

Still to do: Foreign keys are just exported and imported as-is. AC doesn't need this to work, but obviously we should figure out sane special-casing later.
When you say ``raise`` here it immediately re-raises the exception you just caught, so you'll literally never get to the second line here where you set the ``message`` variable.