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
We could change the styling of the flash message so

    .alert {
      color: red
    }

Although we can fix it later.
Or do you mean a different message text?