Integrate Webcal subscriptions

[rfc2822]

Currently Webcal subscriptions are only supported over ICSx⁵. However we want to support them directly in DAVx⁵ as a "Tool" in the Navigation Drawer.

It shall be possible to use the feature next to ICSx⁵, but later it shall replace ICSx⁵ completely.

Webcal subscriptions can be standalone (entered and managed over the Webcal subscriptions activity) or linked to a CalDAV collection (usually managed by the Webcal tab in a CalDAV account).

• DB entity (standalone or be linked to collection_id) + DAO for Webcal subscriptions
• AccountActivity / WebcalFragment: (un)subscribe to Webcal subscription
  • subscription asks whether to subscribe directly in DAVx⁵ or in an external app like ICSx⁵
  • checkbox whether a collection is synced should only check whether it's internally subscribed
  • unsubscription should also only work for internal subscriptions
• WebcalSubscriptionsActivity
• Webcal sync worker
  • should be enabled + called when there's a new subscription
  • should disable itself if there are no active subscriptions