Add dealer checkbox to pre-reg form

Really hacky fix for a last-minute request by the board.

This adds a psuedo-badge called IND_DEALER_BADGE. This badge does not count the attendee as a dealer for is_dealer, because that results in broken behavior (trying to send emails to the group leader, etc). Instead, the badge simply gives the attendee a Dealer ribbon when they register.

In the future, a more generic way to auto-add ribbons based on badge type, or options, would be ideal.