[FEATURE] Synchronize Event Attendees, FreeBusy and Organizers

[bengelhaupt]

This change introduces the ability to also synchronize the attendees of an event including their responses (accept, decline, ...) to the event. This also includes the organizer of such events and whether the event has been marked as being free/transparent or busy/opaque. I added an configuration option too.

There were some pitfalls that had to be considered:

• Google might ignore the organizerAddress and organizerName fields when there are no additional attendees and change them to [email protected] and TheCalendarName.
• Attendee email addresses are automatically lowercased by Google.
• Attendee lists might be reordered by Google.

There also was a bug that would create a reminder in Google Calendar even though it was not supposed to be. Exchange returns a duration in ReminderMinutesBeforeStart, but IsReminderSet also has to be considered because it might be false.

I will add tests if there is an intention to merge this.