Remove old boards from Organisator drop down menu in the event backend

One-sentence description

Remove old boards from Organisator drop down menu in the event backend.

Why?

Old boards don't organise events and the drop down menu is bloated.

Current implementation

When creating a new event, it is possible to choose old boards as organisers.

Desired implementation

When creating a new event, it is not possible to choose old boards as organisers.

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