Don't upload event when calendar is "read-only"

[devvv4ever]

Use case: Users have a read-only calendar (correctly recognized by DAVx5 as read only), for instance the "Contact's birthdays" of Nextcloud. Although the calendar is read-only people may edit the some local properties like "alarm/reminders". In this case the event becomes dirty, but it must NOT be uploaded. Otherwise the server sends an error.

See also https://github.com/bitfireAT/davx5/issues/54

[rfc2822]

• Make readOnly a LocalCollectionProperty
• Lift readOnly from ContactsSyncManager up to SyncManager,
• set accordingly for all collection types
  • LocalAddressBook (readOnly prop existed already)
  • LocalCalendar
  • LocalJtxCollection - can't get/set readOnly prop - assuming write access for now
  • LocalTaskList
• CalendarSyncManager:
  • uploadDirty should handle readOnly accordingly
  • processLocallyDeleted should handle readOnly accordingly

Probably correct handling for the collection types (tasks, jtx entries) would be a good idea, too.