Add 'woocommerce_mail_content' to preview

Added the 'woocommerce_mail_content' filter which is used before sending the email in /includes/emails/class-wc-email.php#L465 to the preview

includes/admin/class-wc-admin.php

@@ -180,7 +180,7 @@ class WC_Admin {
 			$email         = new WC_Email();
 
 			// wrap the content with the email template and then add styles
-			$message       = $email->style_inline( $mailer->wrap_message( $email_heading, $message ) );
+			$message       = apply_filters( 'woocommerce_mail_content', $email->style_inline( $mailer->wrap_message( $email_heading, $message ) ) );
 
 			// print the preview email
 			echo $message;