Fix Rails secrets deprecation warning (Devise)

This is due to heartcombo/devise#5644.

config/initializers/devise.rb

@@ -6,6 +6,7 @@
   # confirmation, reset password and unlock tokens in the database.
   # Devise will use the `secret_key_base` as its `secret_key`
   # by default. You can change it below and use your own secret key.
+  config.secret_key = Rails.application.secret_key_base
 
   # ==> Mailer Configuration
   # Configure the e-mail address which will be shown in Devise::Mailer,