Show booked days in calendar

Thank you jbfelix for your feedback,
Possible, yes. It will require changing the getDays endpoint to call the getSlots for each day.
We'll need to check the impact of those up to 31 (days) extra queries.

Available on 1.1.x for review.
(composer require 'drupal/webform_booking:^1.1')

Tested on 1.1.x. Works as described in the issue description. Instead of adding the class "booked" as suggested by @jbfelix it adds the more descriptive class name "no-slots".