iCloud.com: Manage Task Lists

## Problem

After a bit of research, it seems like Apple shifted gears on iOS 13 how Reminders are synced with iCloud.com because of new features. [This caused quite some trouble](https://github.com/nextcloud/server/issues/17190).

So there might be two different scenarios to deal with: Already upgraded Reminders, and not yet upgraded Reminders: https://support.apple.com/en-us/HT210220

Its a bit nebulous what the implications are. So this issue is to discuss on how we could potentially support task list management (Create, Update, Delete) for iCloud.com

## Prior Art

Ideally we can reuse [the current WebDAV support](https://github.com/elementary/tasks/pull/150) to manage task lists on iCloud.com - but for @danrabbit this does not seem to work at all. On my end, the iCloud account shows up in the Tasks app and although I'm able to add new task lists, those newly created lists are not shown when logged in to iCloud.com