[WIP] Better support for recurrence instances

[geier]

Nothing done yet

This should improve:

• Deleting
  • Deleting all future instances from a specific instance #1122
  • Perhaps: Undelete a deleted instance #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 #750
• Importing
  • importing .ics files that specify only a specific recurrence id should only affect that id #722

Other Related issues:

• #793
• #873

Tasks

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