…zation where relevant

* Move debug code into AMP_Validation_Utils class.
* Remove disable_invalid_removal sanitizer arg in favor of validation_error_callback

* Run amp_content_sanitizers filter late to apply after sanitizers have been added.
* Wrap any existing validation_error_callback functions which have been supplied to do the handling of the validation error.
* Pass flag to style sanitizer to locate_sources to ensure sources are available for reporting when they'll be needed.
* Add handle_validation_error and set_current_node methods to reduce duplication.
* Fix inclusion of property_value in validation_error.
* Pass debug flag to AMP_Validation_Utils upon init.