Allow model config to override sign_in_after_change_password (#5825)

The config exists at the model/resource class from the registerable
module, but it was not being honored, instead we were directly relying
on the main Devise config.

Now this can be configured and honored per-model/resource class, as
expected.

This is similar to #5429 and `sign_in_after_reset_password` fix.

app/controllers/devise/registrations_controller.rb

@@ -163,6 +163,6 @@ class Devise::RegistrationsController < DeviseController
   def sign_in_after_change_password?
     return true if account_update_params[:password].blank?
 
-    Devise.sign_in_after_change_password
+    resource_class.sign_in_after_change_password
   end
 end