Add generation of custom partial discount codes.

[kcomandich]

We'd like to be able to email speakers their free pass, and at the same time send them a link they can give their friends which gives a discount. Currently we are just sending a single friend code used by all speakers, but we could generate custom codes.
Besides generating new codes, this requires a new type of Compticketeer "TicketKind", which is capable of emailing people two different codes at once.
So this is low priority and adds a bit of complexity.