[WIP] Better support for recurrence instances

[geier]

Nothing done yet

This should improve:

• Deleting
  • Deleting all future instances from a specific instance Delete future instances of a recurring event #1122
  • Perhaps: Undelete a deleted instance Can I undelete an instance of a repeating event #1100
• Editing
  • if editing a recurring event, ask if the series or the instance should be edited
  • keep the original start and end date when editing the series Editing a recurring event removes previous instances #750
• Importing
  • importing .ics files that specify only a specific recurrence id should only affect that id Series disappears when one occurence is updated #722

Other Related issues:

• ikhal: Deleting instances of recurring revent yileds AssertionError #793
• Importing updated event breaks database due to duplicate UID #873

Tasks

• ability to get the complete .ics for the whole series when getting only one instance