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/managers/class-email-manager.php

@@ -383,6 +383,9 @@ class Email_Manager extends Base_Manager {
 	 * @return void
 	 */
 	public function register_all_default_system_emails(): void {
+
+		// TODO: Don't render every email until they are used.
+
 		/*
 		 * Payment Successful - Admin
 		 */