[Admin bypass] Skip confirmation when "allow bypass of placeholder user confirmation" is enabled

⚠️ We should ask AppSec (@ameyadarshan) to review the MRs that implement the confirmation skip to make sure we don't accidentally implement a loophole

Add logic to skip placeholder user reassignment confirmation when setting is enabled.

All of these three criteria must evaluate to true when checking if we should bypass:

• The instance is currently configured to allow user impersonation, and
• The bypass admin setting is enabled, and
• The user is an admin (use User#can_admin_all_resources?, which works with admin mode)

Event tracking

Add an event for the action of admin reassigning contributions from placeholder to user without confirmation. See !182679 (merged) for how it's done.