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/widget/notes_widget.dart

@@ -0,0 +1,81 @@
+import "package:flutter/material.dart";
+import "package:font_awesome_flutter/font_awesome_flutter.dart";
+import "package:inventree/inventree/model.dart";
+import "package:inventree/widget/refreshable_state.dart";
+import "package:flutter_markdown/flutter_markdown.dart";
+import "package:inventree/l10.dart";
+
+
+/*
+ * A widget for displaying the notes associated with a given model.
+ * We need to pass in the following parameters:
+ * 
+ * - Model instance
+ * - Title for the app bar
+ */
+class NotesWidget extends StatefulWidget {
+
+  const NotesWidget(this.model, {Key? key}) : super(key: key);
+
+  final InvenTreeModel model;
+
+  @override
+  _NotesState createState() => _NotesState();
+}
+
+
+/*
+ * Class representing the state of the NotesWidget
+ */
+class _NotesState extends RefreshableState<NotesWidget> {
+
+  _NotesState();
+
+  @override
+  Future<void> request(BuildContext context) async {
+    await widget.model.reload();
+  }
+
+  @override
+  String getAppBarTitle() => L10().editNotes;
+
+  @override
+  List<Widget> appBarActions(BuildContext context) {
+
+    List<Widget> actions = [];
+
+    if (widget.model.canEdit) {
+      actions.add(
+        IconButton(
+          icon: FaIcon(FontAwesomeIcons.penToSquare),
+          tooltip: L10().edit,
+          onPressed: () {
+            widget.model.editForm(
+              context,
+              L10().editNotes,
+              fields: {
+                "notes": {
+                  "multiline": true,
+                }
+              },
+              onSuccess: (data) async {
+                refresh(context);
+              }
+            );
+          }
+        )
+      );
+    }
+
+    return actions;
+  }
+
+  @override
+  Widget getBody(BuildContext context) {
+    return Markdown(
+      selectable: false,
+      data: widget.model.notes,
+    );
+  }
+
+}