Sales order support (#438)

* Add new models for SalesOrder

- Create generic Order and OrderLine models with common functionality

* Refactor

- Move some widgets around
- Cleanup directory structure

* Add link to home screen and nav drawer

* Add SalesOrder list widget

* Linting fixes

* Fix string

* Refactor PurchaseOrderDetailWidget

* Tweaks to existing code

* linting

* Fixes for drawer widget

* Add "detail" page for SalesOrder

* Add more tiles to SalesOrder detail

* Allow editing of salesorder

* add list filters for sales orders

* Display list of line items

* Customer updates

- Display customer icon on home screen
- Fetch sales orders for customer detail page

* Cleanup company detail view

* Create new sales order from list

* Stricter typing for formFields method

* Create new PurchaseOrder and SalesOrder from company deatil

* Status code updates

- Add function for name comparison
- Remove hard-coded values

* Update view permission checks for home widget

* Add ability to manually add SalesOrderLineItem

* Add nice progress bar widgets

* Display detail view for sales order line item

* edit SalesOrderLineItem

* Fix unused import

* Hide "shipped items" tab

- Will be added in a future update

lib/inventree/company.dart

@@ -22,7 +22,7 @@ class InvenTreeCompany extends InvenTreeModel {
   List<String> get rolesRequired => ["purchase_order", "sales_order", "return_order"];
 
   @override
-  Map<String, dynamic> formFields() {
+  Map<String, Map<String, dynamic>> formFields() {
     return {
       "name": {},
       "description": {},
@@ -121,8 +121,8 @@ class InvenTreeSupplierPart extends InvenTreeModel {
   List<String> get rolesRequired => ["part", "purchase_order"];
 
   @override
-  Map<String, dynamic> formFields() {
-    Map<String, dynamic> fields = {
+  Map<String, Map<String, dynamic>> formFields() {
+    Map<String, Map<String, dynamic>> fields = {
       "supplier": {},
       "SKU": {},
       "link": {},