Added IPN to Supplier Part string representation

Improved Supplier Part templates and fixed 'Order Part' button in supplier_part_orders

InvenTree/order/templates/order/purchase_order_detail.html

@@ -13,7 +13,7 @@
 
 <div id='order-toolbar-buttons' class='btn-group' style='float: right;'>
     {% if order.status == PurchaseOrderStatus.PENDING and roles.purchase_order.change %}
-    <button type='button' class='btn btn-default' id='new-po-line'>{% trans "Add Line Item" %}</button>
+    <button type='button' class='btn btn-primary' id='new-po-line'>{% trans "Add Line Item" %}</button>
     {% endif %}
 </div>