Check for updates and update public calendars

Fetch the open calendars every 20 seconds
and check if they have changed since they were loaded.

If a calendar changed remove all it's data from the store
and thereby effectively trigger a reload.

This is a fairly brute force approach as it drops all known data
rather than fetching only the data that changed via the caldav sync API.

However Caldav sync is yet to be implemented. (nextcloud/cdav-library#9)
So this serves as a workaround for the time being.

See #31.

Signed-off-by: Azul <azul@riseup.net>