Wrap dpa_feedback_achievement_unlock_wrapper in function_exist checks

If the unlock feedback shortcode is used directly, the enqueue script
methods (to be added in a future commit), load this file again -- and
PHP errors because the function already exists.
Use wrapper functions rather than accessing DPA_Theme_Compat proeprties directly
Store reference to the DPA_Default theme class in the achievements object.
When manually using the user notifications shortcode, enqueue notifications style/script.