Create calendar analyzer for availabilities

https://trello.com/c/FulwCRAw/26-refactor-availability-and-pickup-methods-with-new-object
extract into private method, then reformat as:

``` ruby
private

def within_range(range)
  @availabilities
    .where(date: range)
    .group_by(&:date)
    .sort_by { |date, _| date }
end
```