title: Add calculated saving total in Woocommerce order totals rows
body: <p>In Woocommerce, I am using the following code to calculate and display 'Total Saving' on the order in Cart and checkout pages: </p>
  
  <pre><code>function wc_discount_total_30() {
      global $woocommerce;
      $discount_total = 0;
      foreach ( $woocommerce-&gt;cart-&gt;get_cart() as $cart_item_key =&gt; $values) {
          $_product = $values['data'];
          if ( $_product-&gt;is_on_sale() ) {
              $regular_price = $_product-&gt;get_regular_price();
              $sale_price = $_product-&gt;get_sale_price();
              $discount = ($regular_price - $sale_price) * $values['quantity'];
              $discount_total += $discount;
          }
      }
      if ( $discount_total &gt; 0 ) {
          echo '&lt;tr class="cart-discount"&gt;
          &lt;th&gt;'. __( 'Saved', 'tsavedis' ) .'&lt;/th&gt;
          &lt;td data-title=" '. __( 'Saved', 'tsavedis' ) .' "&gt;'
          . wc_price( $discount_total + $woocommerce-&gt;cart-&gt;discount_cart ) .'&lt;/td&gt;
          &lt;/tr&gt;';
      }
  }
  
  // Hook our values to the Basket and Checkout pages
  add_action( 'woocommerce_cart_totals_after_order_total', 'wc_discount_total_30', 99);
  add_action( 'woocommerce_review_order_after_order_total', 'wc_discount_total_30', 99);
  </code></pre>
  
  <p>I need to display this total saving in 'Order Edit' page at backend as a custom field.<br>
  How to do that ?</p>