Send informative email for already confirmed users

Currently the application does not send any email to confirm the
account for already confirmed users. But we show a notice message
that may look like you will recive one:

"If your email address exists in our database, you will receive
 an email with instructions for how to confirm your email address
 in a few minutes."

In this commit we keep the original message, but send an email to
the user informing them that their account is now registered.

This way no one can know if someone else's account is confirmed and
we don't have to worry about GDPR either.

Co-Authored-By: taitus <sebastia.roig@gmail.com>

app/controllers/users/confirmations_controller.rb

@@ -1,4 +1,17 @@
 class Users::ConfirmationsController < Devise::ConfirmationsController
+  # POST /resource/confirmation
+  def create
+    self.resource = resource_class.send_confirmation_instructions(resource_params)
+    yield resource if block_given?
+
+    if successfully_sent?(resource)
+      Mailer.already_confirmed(resource).deliver_later unless resource.confirmation_required?
+      respond_with({}, location: after_resending_confirmation_instructions_path_for(resource_name))
+    else
+      respond_with(resource)
+    end
+  end
+
   # new action, PATCH does not exist in the default Devise::ConfirmationsController
   # PATCH /resource/confirmation
   def update