Prep Plugin for release on WordPress.org

Escape everything that should be escaped.
Add nonce checks where needed.
Sanitize all inputs.
Apply Code style changes across the codebase.
Correct many deprecation notices.
Optimize load order of many filters.

inc/functions/financial.php

@@ -358,7 +358,7 @@ function wu_calculate_taxes_by_day($start_date = false, $end_date = false, $incl
 
 	foreach ($line_items_groups as $line_items_group) {
 		foreach ($line_items_group as $line_item) {
-			$date = gmdate('Y-m-d', strtotime((string) $line_item->date_created));
+			$date = gmdate('Y-m-d', strtotime($line_item->get_date_created()));
 
 			if ( ! wu_get_isset($data, $date)) {
 				$data[ $date ] = [