Create reservations functionality

Nest reservation route inside listing
Add has_many relationship to user model
Create DateRange model
Create reservation controller
Create link to reservation in listing show
Create new reservation form
Add belongs_to relationship listing
You could move the `valid_dates?` logic into the `DateRange` model.

```
# date_range.rb
class DateRange
  def valid?
    # implement here
  end
end

# reservations_controller.rb
range = DateRange.new(...)
if range.valid?
  @reservation = ...
end
```