Add support for company attachments (#261)

* Add support for company attachments

- Add API version check
- Add new class
- Add link to company detail page
- Assorted refactoring

* linting fixes

lib/inventree/company.dart

@@ -86,6 +86,26 @@ class InvenTreeCompany extends InvenTreeModel {
 }
 
 
+/*
+ * Class representing an attachment file against a Company object
+ */
+class InvenTreeCompanyAttachment extends InvenTreeAttachment {
+
+  InvenTreeCompanyAttachment() : super();
+
+  InvenTreeCompanyAttachment.fromJson(Map<String, dynamic> json) : super.fromJson(json);
+
+  @override
+  String get REFERENCE_FIELD => "company";
+
+  @override
+  String get URL => "company/attachment/";
+
+  @override
+  InvenTreeModel createFromJson(Map<String, dynamic> json) => InvenTreeCompanyAttachment.fromJson(json);
+
+}
+
 /*
  * The InvenTreeSupplierPart class represents the SupplierPart model in the InvenTree database
  */