Added ability to use discount codes on reservation page.
Related to #27. Still needs to be added to other pages and we need to take used codes into account in price display and display active codes in cart sidebar.
Fixed collapsing panel appearance.

Bootstrap only makes collapsing panels look good if they're
part of a panel-group.
Added basic formset javascript.

JS provided by zenaida as of littleweaver/django-zenaida@f27ceb1b579c57e340aa112fa626b9d187ad59bc
Item option formset to renumber items.