Add download ical file for events

[allc]

Implements #20

Button like this

start_ and end_ added in constants/calendarevents.ts as timezone is a pain to deal with when using event._instance.range passed to event handler from FullCalendar

[tomasmaillo]

Ooooh this is so fun. Must be very useful for mobile use too.

Is there any way we can make it also generate a Google Calendar link too? If you click on it, it should take you to Google Calendar and auto populate regions.

Unless you are in the know about .ics files, I would imagine most users to be extremely confused when a random file gets downloaded.

[allc]

There is a https://add-to-calendar-button.com/ "Add to Calendar Button" component which support multiple platforms, as well as .ics maybe something would be preferred to look into in this case?

Though I don't see it's a problem, be aware it uses Elastic License 2.0 https://github.com/add2cal/add-to-calendar-button?tab=readme-ov-file#-copyright-and-license