Export function for all attendee raw data

Added a all_attendees function to /summary that exports every piece of
information saved in the database. Some of the information is too raw
for human use.
Added attendee_upload function

Adds a way to upload a .csv to import attendees. Currently doesn't work
100% correctly for all data.
Refactor export and import to work dynamically

Exporting and importing now works on all columns in the Attendee
database without needing to special-case specific columns. It instead
modifies them based on their type.

Foreign keys are just exported as keys, but nothing breaks horribly.
Minor fixes for attendee import

Removes an extra 'raise' that caused an infinite loop, and orders
exported attendees by badge number.
Display attendees after upload

Displays a list of attendees imported via the upload function. Also
allows the user to select a date format, and fixes a couple issues with
sessions getting messed up.
Merge pull request #760 from magfest/import_attendees

Export and Import Attendees