Internet Calendars (ICS calendars)

Basically the same request as https://amplenote.featureupvote.com/suggestions/359301/allow-for-internet-calendars

https://en.wikipedia.org/wiki/ICalendar

Should be able to subscribe to public calendars, but also publish amplenote calendars so they can be shared without linking accounts.

My primary use case is proton calendar for example only has internet calendars so doesn't work well in combination with amplenote.