Refactor Setting (#3302)

* @object is not needed

* Remove unneeded dependencies

* Do not call private method

* Prefer #respond_to_missing? over #respond_to?

`#respond_to?` doesn't support `User.settings.method(:method_name)`

* Use find_or_initialize_by instead of

app/models/setting.rb

@@ -23,7 +23,7 @@ class Setting < RailsSettings::Base
     def [](key)
       return super(key) unless rails_initialized?
 
-      val = Rails.cache.fetch(cache_key(key, @object)) do
+      val = Rails.cache.fetch(cache_key(key, nil)) do
         db_val = object(key)
 
         if db_val
@@ -50,8 +50,6 @@ class Setting < RailsSettings::Base
       records
     end
 
-    private
-
     def default_settings
       return {} unless RailsSettings::Default.enabled?
       RailsSettings::Default.instance