Add iCal support for the timetable.

[anpolimus]

At the Program page (https://drupalcampkyiv.org/en/node/7) we have ability to add reminder to your google calendar.

Our community requested support for Apple devices (iCal).

Information about iCal format: https://apple.stackexchange.com/questions/125338/calendar-ical-ics-format

Plan.

1. At our field formatter we have to add iCal download link in addition to existed google calendar.
   https://github.com/drupal-ukraine/dcamp-site/blob/master/docroot/modules/custom/dckyiv_core/src/Plugin/Field/FieldFormatter/TimetablePresentationFormatter.php#L73

2. Add custom controller that will get internal parameters about dates and additional info and sends ics file to user.

3. Display and style (if needed) link at the timetable block.

Additional info: iCal file generation you could find at this D7 contrib source code:
https://git.drupalcode.org/project/addtocal/blob/7.x-1.x/addtocal.module#L583