users: use sudo mode when changing email addresses.

When a user tries to change their email, redirect them to the confirm
password page (like Github's sudo mode) instead of having them re-enter
their password on the change email page. This is the same thing we do
when a user updates their API keys. This way we have can use the same
confirm password authentication flow for everything that needs a
password.

app/models/user.rb

@@ -361,6 +361,17 @@ class User < ApplicationRecord
     def can_receive_email?(require_verification: true)
       email_address.present? && email_address.is_deliverable? && (email_address.is_verified? || !require_verification)
     end
+
+    def change_email(new_email, request)
+      transaction do
+        update(email_address_attributes: { address: new_email })
+
+        if errors.none?
+          UserEvent.create_from_request!(self, :email_change, request)
+          UserMailer.email_change_confirmation(self).deliver_later
+        end
+      end
+    end
   end
 
   concerning :BlacklistMethods do