Add Reservation to Rentr

* Users can create reservation dates
* Users can not create invalid reservation date ranges
* Users can view their reservations for a particular listing
* Users can view all of their reservations
* Users can view all their reservations for a listing if any exist
* Guests can still use app
No need for `presence:` or `Proc.new` here.

`validates :start_date, date: { before: :end_date }`

You can't have a one-day reservation, so the validation should be `before` not `before_or_equal_to`.