Add internal event tracking to placeholder reassignment

What does this MR do and why?

This adds internal event tracking to the following events in placeholder reassignment:

• placeholder user created (action leading to pending_reassignemnt)
• placeholder reassignment proposed (action leading to awaiting_approval)
• placeholder reassignment accepted (action leading to reassignment_in_progress)
• placeholder_reassignment_canceled
• placeholder reassignment rejected
• placeholder reassignment failed
• keep as placeholder user
• keep all as placeholder
• placeholder reassignment completed

MR acceptance checklist

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Related to #514952 (closed)