Handle case that user wants to change his email into one that already exists

[fynnlyte]

Users occasionally want to change their email into an address with which they have already registered an account.

In that case, the email change requests indicates a success (because we don't allow account guessing) but nothing actually happens. The user is left in the dark.

Possible solution: Similar to the handling of the registration flow, send out an informational email that there is already an account associated with that email to the new email only. This way, the user can be informed that this account would need to be deleted before they can change into the mail.