Rework Cart UX

[orenyk]

Inspired by @squidgetx's comment in #1114, we need to improve the workflow for the cart / creating reservations. As things stand, we're sometimes pre-empting our validations with our JS / UI elements (e.g. disabling the "Add to Cart" button when the additional availability during the current cart dates is zero, but only in the catalog), and it's possible that we could significantly improve the user experience by decoupling the selection of equipment from the selection of dates. This will be a big bucket issue for discussion (some of which might occur offline).

[orenyk]

Inspired by #1131, we need to clarify how we handle and communicate the failure of validations, both "hard" (i.e. ActiveRecord) and "soft" (i.e. Cart).