Project code support (#336)

* Determine if project codes are supported

* Add helpers for boolean functions

* Adds helper methods for generic "model" class

- Will allow us to do some good refactoring

* Refactor the refactor

* Add debug support and getMap function

* Major refactoring for model data accessors

* Handle null values

* Add sentry reporting if key is used incorrectly

* Fix typo

* Refactor createFromJson function

* Add model for ProjectCode

* Display and edit project code for purchase orders

lib/inventree/project_code.dart

@@ -0,0 +1,28 @@
+import "package:inventree/inventree/model.dart";
+
+
+/*
+ * Class representing the ProjectCode database model
+ */
+class InvenTreeProjectCode extends InvenTreeModel {
+
+  InvenTreeProjectCode() : super();
+
+  InvenTreeProjectCode.fromJson(Map<String, dynamic> json) : super.fromJson(json);
+
+  @override
+  InvenTreeModel createFromJson(Map<String, dynamic> json) => InvenTreeProjectCode.fromJson(json);
+
+  @override
+  String get URL => "project-code/";
+
+  @override
+  Map<String, dynamic> formFields() {
+    return {
+      "code": {},
+      "description": {},
+    };
+  }
+
+  String get code => getString("code");
+}