Remove deprecation warning

It's coming from Devise and can be removed by specifying the
secret_key_base explicitly:
heartcombo/devise#5644

config/initializers/devise.rb

@@ -14,7 +14,7 @@
   # 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 = ''
+  config.secret_key = Rails.application.secret_key_base
 
   # ==> Controller configuration
   # Configure the parent class to the devise controllers.