Only allow a single t-shirt to be ordered.

In the original version, the user would be allowed to add however many t-shirts he/she wants to the basket. Upon completing the order, the user would then be notified that only one (free) t-shirt can be ordered.

This pull request contains changes that will refuse adding tshirts to the basket once one is already placed in the basked. In this case, the user is notified of this with a message and in the iOS app, the animation from the order button to the basket is reversed and the circle is set to red instead of green.