Integrate EventKit

[rigrergl]

• user can add an in-app event to their apple calendar
• prevent adding events twice

For adding events, use this tutorial:
https://sodocumentation.net/ios/topic/5854/eventkit

For preventing adding events twice, consider a simple title and time match:
https://developer.apple.com/documentation/eventkit/retrieving_events_and_reminders?language=objc