Adapt to devise 3.2 and use after_confirmation callback to send welcome email

app/models/spree/user.rb

@@ -34,10 +34,6 @@ module Spree
 
     # We use the same options as Spree and add :confirmable
     devise :confirmable, reconfirmable: true
-    # TODO: Later versions of devise have a dedicated after_confirmation callback, so use that
-    after_update :welcome_after_confirm, if: lambda {
-      confirmation_token_changed? && confirmation_token.nil?
-    }
 
     class DestroyWithOrdersError < StandardError; end
 
@@ -82,9 +78,9 @@ module Spree
       customers.find_by(enterprise_id: enterprise)
     end
 
-    def welcome_after_confirm
-      # Send welcome email if we are confirming an user's email
-      # Note: this callback only runs on email confirmation
+    # This is a Devise Confirmable callback that runs on email confirmation
+    # It sends a welcome email after the user email is confirmed
+    def after_confirmation
       return unless confirmed? && unconfirmed_email.nil? && !unconfirmed_email_changed?
 
       send_signup_confirmation