Calendar: Hide duplicate events / combine them

[georgehrke]

see https://github.com/nextcloud/server/issues/12501#issuecomment-503612800

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[georgehrke]

Different aspects to consider when selecting the event to show:

• highest permission
• LAST-MODIFIED property
• If it's an invitation, is it my copy

[georgehrke]

The editor should also show a message that this event is present in Calendar A, Calendar B and Calendar C ...

[Defcon0]

This is definitely needed. We are currently migrating to Nextcloud calendar but this is the most annoying thing to most users.

[georgehrke]

Please use the Github Reactions feature instead of commenting +1, 👍 etc. These kinds of comments just unnecessarily notify everyone subscribed to this issue. Thx! :)