Shirt color+type support

Kinda WIP.
Kinda hacky.
Kinda throwaway just for Magstock to work.


The key bit here is the following:
- added new DB field attendee.shirt_color  (black vs white for tie dye)
- re-purposing attendee.amount_extra so it can only be used for shirt (since supporters are disabled)

I need to do a bit more testing of this to make sure sane things are getting saved in the DB when you edit attendees from the admin side.  I'll definitely be adding another commit or two to this changelist.

I'm not terribly proud of this code but it'll get the job done.  In future ubers we could revisit these concepts.