Product attributes should be selected from a list

[russss]

We have the concept of attributes on products. These are currently set by typing JSON into a text box when you create or edit a product in the admin interface. This is error-prone, there should be checkboxes instead.

There are three attributes that we use currently:

• is_transferable to determine whether a product can be transferred
• has_badge to determine whether a ticket entitles a user to a badge
• requires_admission_ticket which requires the user to have an admission ticket in order to buy this item (defaults to True)