Updating gender choices

This has fixes to migrater.py and db.sql - if you update your branch from swcarpentry/master, most of the noise should go away.

However, is it possible to display `unknown` instead of `------` when the person's gender is unknown?

Also, the airport selector initially shows `--------` for some (most) people.  When the form is submitted, the error message 'required field' appears next to it - we actually don't know airports (locations) for most of the people in the database, so 'unknown' needs to be a valid option there as well.