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/part.dart

@@ -23,6 +23,9 @@ class InvenTreePartCategory extends InvenTreeModel {
   @override
   String get URL => "part/category/";
 
+  @override
+  List<String> get rolesRequired => ["part_category"];
+
   @override
   Map<String, dynamic> formFields() {
 
@@ -182,6 +185,9 @@ class InvenTreePart extends InvenTreeModel {
   @override
   String get URL => "part/";
 
+  @override
+  List<String> get rolesRequired => ["part"];
+
   @override
   Map<String, dynamic> formFields() {
     return {