Update statistics fields
Update statistics fields

 * Remove 'occupation', it's too free-form to be useful
 * Add "How did you hear about GUADEC?" option, so we can get an idea
   if any of our local promotion has any effect
 * Make "country" a list of countries (with "unspecified" as the
   default) and clarify that it's country of residence

thanostx/regcfp,thanostx/regcfp,thanostx/regcfp