Track created_by on orders

This work is building up to a fix for #3346
last_incomplete_spree_order for a User will now only return orders created by that user

Fixes #3346
[backend] Add missing stub for find_by_number in OrdersControllerSpec
[frontend] Stub created_by creation in orders_controller_spec
[api] Correct line item check in orders_controller_spec
Fix Spree::Calculator.calculators method

Fixes #3426
Make zones more robust by allowing types with 2 words (i.e. zip code).

Fixes #3431
Add #total_on_hand method to Variant model

Fixes #3427
[api] Correct line item quantity checking in orders_controller_spec