Reinstate wh_supported_post_types filter

Allow WH to be enabled for a post type by `add_post_type_support()` or by `wh_supported_post_types` filter. Previous commit ignored values added via the filter in order to fix error when filtered list contained invalid post type. However, it looks like WPCOM is still using an older version of the plugin that includes the `wh_supported_post_types` filter.