Refs #654 -- API: Writable item endpoints (#676)

* MKBDIGI-184: Basic create added for API items endpoint

* MKBDIGI-184: Starting endpoint for GET /api/v1/organizers/(organizer)/events/(event)/items/(id)/variations/

* MKBDIGI-184: endpoint for GET /api/v1/organizers/(organizer)/events/(event)/items/(id)/variations/

* MKBDIGI-184: Completed endpoint for variations

* MKBDIGI-184: Added endpoint for addons

* MKBDIGI-184: Added Item validation

* MKBDIGI-184: Added check for order/cart positions on item variation destroy.

* MKBDIGI-184: Fixed check for order/cart positions on item variation destroy.

* MKBDIGI-184: Updated tests, validation for addons

* MKBDIGI-184: Documentation feedback corrections

* MKBDIGI-184: Added documentation for item add-ons

* MKBDIGI-184: Code formatting fixes

* MKBDIGI-184: Feedback fixes

* MKBDIGI-184: Updated tests for delete item

* MKBDIGI-184: Cleaned up tests

* MKBDIGI-184: Added additional test URLs

* MKBDIGI-184: Documentation fixes

* MKBDIGI-184: Fixed read-only fields/Documentation

* MKBDIGI-184: Documentation fixes

* MKBDIGI-184: Added helper for dict merge for 3.4 compatibility

* MKBDIGI-184: Validation updates

* MKBDIGI-184: Fixed permissions test error. Changed to HTTP 404 for POST to addons endpoint

* MKBDIGI-184: Implemented nested variations and add-ons for POST on the item endpoint.

src/tests/api/conftest.py

@@ -117,3 +117,8 @@ def subevent2(event2, meta_prop):
 @pytest.fixture
 def taxrule(event):
     return event.tax_rules.create(name="VAT", rate=19)
+
+
+@pytest.fixture
+def taxrule2(event2):
+    return event2.tax_rules.create(name="VAT", rate=25)