Itinerary Cart

This implements the cart for customers on the explore page. It should be able to:

1. Be empty
2. Update as users add items to trip
3. Delete items from trip
4. Automatically switch to current trip based on previous input
5. Affix to side
6. Toggle properly on small screens (thank to @thejinxters for the help!)
7. Affix to top when scrolling down.

Boom-shacka-lacka :basketball: