[Fix Cart Latency] Remove CartReservations!

This PR removes CartReservations entirely from the project. Most calls to CartReservations have been replaced with cart.prepare_all, which builds reservations out of the contents of the cart without needing any interaction with the database
agreed, it's currently hard-coded into the factory as 10 days (see `/spec/factories/equipment_models.rb`) but better to refer to the equipment model rather than an arbitrary constant.