Otherwise we'd be mistakenly displaying the original email in the
message (which is the same we're sending the message to).
Also tweak the messaging a bit in this case, to show that the email "is
being changed" (the change hasn't taken effect yet).
Related to #4455.
This better indicates what the setting is for, and when it's supposed to
be triggered.
We might eventually deprecate the existing password_change on in favor
of password_changed.
This adds a new setting `send_email_change_notification` which will
send an email to the original user email when their email is updated to
a new one.
It doesn't take into account the reconfirmable setting yet, it will be
added next, so that if confirmable is included and reconfirmable is
being used, the email will be triggered when the email change is
requested, not when confirmed (e.g when we store the email in
`unconfirmed_email`, not when it's later copied to `email` when that is
confirmed).
