Fix email confirmation link not updating email (#6187)

A change introduced in #6125 prevents
`Devise::Models::Confirmable#confirm` from being called for existing
users, which in turn leads to `email` not being set to
`unconfirmed_email`, breaking email updates. This also adds a test
that would've caught this issue.

app/models/user.rb

@@ -126,18 +126,18 @@ class User < ApplicationRecord
   end
 
   def confirm
-    return if confirmed?
+    new_user = !confirmed?
 
     super
-    update_statistics!
+    update_statistics! if new_user
   end
 
   def confirm!
-    return if confirmed?
+    new_user = !confirmed?
 
     skip_confirmation!
     save!
-    update_statistics!
+    update_statistics! if new_user
   end
 
   def promote!