Company attachment (#4346)

* Adds new model, API, serializers, etc

- Refactor InvenTreeAttachmentSerializer class
- Reduces code duplication

* Update front end

* Increment API version

InvenTree/users/models.py

@@ -130,6 +130,7 @@ class RuleSet(models.Model):
         ],
         'purchase_order': [
             'company_company',
+            'company_companyattachment',
             'company_manufacturerpart',
             'company_manufacturerpartparameter',
             'company_supplierpart',
@@ -142,6 +143,7 @@ class RuleSet(models.Model):
         ],
         'sales_order': [
             'company_company',
+            'company_companyattachment',
             'order_salesorder',
             'order_salesorderallocation',
             'order_salesorderattachment',