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/notification.dart

@@ -27,8 +27,8 @@ class InvenTreeNotification extends InvenTreeModel {
     };
   }
 
-  String get message => (jsondata["message"] ?? "") as String;
-
+  String get message => getString("message");
+  
   DateTime? get creationDate {
     if (jsondata.containsKey("creation")) {
       return DateTime.tryParse((jsondata["creation"] ?? "") as String);