Make notes widget "generic" (#327)

* Make notes widget "generic"

- No longer tied to the "part" model
- Will allow us to use it elsewhere

* Update release notes

* Add helper methods for checking model permissions

* Refactoring of permissions checks

* Add notes to the "purchase order" widget

* Fix typos

* remove bom tab from part view

* linting fixes

lib/inventree/purchase_order.dart

@@ -18,6 +18,9 @@ class InvenTreePurchaseOrder extends InvenTreeModel {
   @override
   String get URL => "order/po/";
 
+  @override
+  List<String> get rolesRequired => ["purchase_order"];
+
   String get receive_url => "${url}receive/";
 
   @override