As a user with multiple user accounts, I want to have them combined (User Account Combining UAC)

Separate into two use cases:

1. User initiated combining. Make ample sanity checks, escalate anything sus to admin.
2. Admin initiated combining. Show sanity check results but allow the Force to run its natural course.

Perhaps support a hands-free UAC only in the case where the user controls all the email addresses involved: send a confirmation email to each and only perform after each email has confirmed. If any of them denies or takes too long to confirm, escalate.

Sussiness may include (not limited to)
* having wildly different personal details in the accounts (eg. three of these four mismatch: first name, last name, email address, phone number)
* being involved in the same event with both accounts
* having Desuprofile connections for both accounts
* having a prior UAC escalation

Open questions:
* Should we support multiple email addresses? Add an EmailAddress model despite keeping one of them as `Person.email`?
* Should this go through Survey? This would warrant implementation of non-Event surveys and the root Scope `kompassi`, though.