Provide checkbox for "swag" order

[rjschwei]

It would be nice to have a check box that allows registrants to order a t-shirt to be paid for at checking at the event.

The visibility of the check box should be configurable per event and the default should be "I do not want to purchase the conference shirt". When the user un-checks the box the sizes are shown XS, S, M, L, XL, XXL and the price is shown as well. The price also needs to be configurable in the admin interface.

[bear454]

@rjschwei can we close this issue, using the question builder?

[rjschwei]

Probably yes. We'll see when the next conference comes around if it works as we would like it to ;)