Ignore group members in user quick actions

What does this MR do and why?

This MR addresses some of the edge cases with user extraction from parameters in quick-actions.

In particular we:

• guarantee never to return groups from the reference extractor in quick actions when looking for users
• guarantee never to return unreferencable users
• provide more helpful messages, indicating what arguments were rejected

Related issues:

• #352418 (closed) (closed customer bug report)
• #352426 (closed)

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