Calendar feed of schedule entries for phone lock screen integration

Idea by Simba:
It would be nice if it was possible to "connect" a camp to e.g. a Google Calendar. This is partly for convenience because it feels like one is not always logged in to eCamp on the phone (surely depends on the specific user, and may have been improved with #7942). But the main benefit of calendar integration would be to have schedule entries displayed or as notifications on a phone lock screen.

This connection should preferably be per camp, so that camps which are opened for testing or in which the user is only a coach or a cook don't automatically trigger notifications.