Add Recurrence-ID to the events/components by default

[niccokunzmann]

We removed the Recurrence-ID because it seemed unnecessary and clutters the output.
However when you would like to import events into your calendar, the Recurrence-ID is of high importance.

• importing just one event and later updating it, the Recurrence-ID identifies the event
• editing events - to edit events, generally, this information should be available

Tests

• event without repetition does not have a recurrence id
• imported events with recurrence id keep their recurrence ids
• recurrence ID is sanitized to fit type as described above, depending on the event with the recurrence rule
• matrix:
  • Recurrence id (type)
    • date
    • datetime
    • datetime UTC
    • datetime timezone
  • event in series:
    • first event
    • event in rrule
    • event in rdate
    • event moved
    • event edited at same time

Discussion

At the moment, we set keep_recurrence_attributes = False to delete recurrence information. This refers mostly to the RRULE, EXDATE and RDATE and also include the recurrence id.

After this:

• keep_recurrence_attributes = False deletes RRULE, EXDATE and RDATE
• keep_recurrence_attributes = True retains RRULE, EXDATE and RDATE

RECURRENCE-ID will be included no matter what and by default because it is a detail that requires understanding and by default the most useful choice for beginners should be chosen. This is to allow import/export of events into calendars for which the recurrence ID is required.

• Increase MINOR version number

Context

• Add Recurrence ID to downloaded events open-web-calendar#651

Questions

If you require removing the Recurrence ID by default, then please comment.
Any other comments on this are very welcome! Especially if you use the library in production in your system and this change will impact you.