Shopping basket mode

[will-r]

Some changes to make the entry process feel more manageable and to allow people to buy entries for other people:

• show options at the top of the form: for me, for someone else
• somehow find a way to assign entries to existing competitors without showing their information to just anybody.
• or alternatively, work like sportident; allow people to create accounts for other people and remember the association for next time. Next time Hazel Robinson sees a 'Mike' option. But what if Mike also signs in?
• finish the individual race entry process with an 'add to basket' button. Click that and the form updates to say 'checkout now' and 'create another entry'. We also show a little basket icon next to the user blob, which I guess is just bound to a basket object.

I think this involves only two new models:

• basket
• transaction

since the basket can only contain entries, and the quantity is always 1, we don't need line items and all that.