Rerender broadcast preview when type changes

What does this MR do?

Reloads the broadcast message preview from the "backend" when the broadcast type changes. We need this for broadcast message placeholders. The placeholders will only be available for notifications and we'd otherwise not get this information in broadcast_message_helper when re-rendering.

Before this change, we only refreshed the preview from the "backend" when the text changes.

Part of: #211369 (closed)

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Does this MR meet the acceptance criteria?

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