[Spree 2 Upgrade] Restore Variant.on_hand methods

[mkllnk]

Description

Working on #2014 we found that we would like to restore the old Spree API to access and modify count_on_hand values. Even though that database column doesn't exist any more, we can pass all calls to the new default stock location.

Expected Behavior

OFN code can still use the following methods on Variant:

• count_on_hand
• count_on_hand=
• on_hand
• on_hand=

Actual Behavior

With Spree 2, these methods are gone and our calls fail.

Steps to Reproduce

The following specs fail explicitly because of on_hand methods missing. But a lot of them because of Spree::Product.on_hand, not Varaint.

./spec/controllers/admin/proxy_orders_controller_spec.rb
./spec/controllers/admin/subscriptions_controller_spec.rb
./spec/controllers/cart_controller_spec.rb
./spec/controllers/spree/admin/products_controller_spec.rb
./spec/controllers/spree/admin/variants_controller_spec.rb
./spec/controllers/spree/orders_controller_spec.rb
./spec/features/admin/adjustments_spec.rb
./spec/features/admin/bulk_product_update_spec.rb
./spec/features/admin/enterprises_spec.rb
./spec/features/admin/order_cycles_spec.rb
./spec/features/admin/orders_spec.rb
./spec/features/admin/product_import_spec.rb
./spec/features/admin/products_spec.rb
./spec/features/admin/reports_spec.rb
./spec/features/admin/variant_overrides_spec.rb
./spec/features/admin/variants_spec.rb
./spec/features/consumer/groups_spec.rb
./spec/features/consumer/producers_spec.rb
./spec/features/consumer/shopping/cart_spec.rb
./spec/features/consumer/shopping/checkout_auth_spec.rb
./spec/features/consumer/shopping/checkout_spec.rb
./spec/features/consumer/shopping/embedded_shopfronts_spec.rb
./spec/features/consumer/shopping/products_spec.rb
./spec/features/consumer/shopping/shopping_spec.rb
./spec/features/consumer/shopping/variant_overrides_spec.rb
./spec/features/consumer/shops_spec.rb
./spec/jobs/subscription_confirm_job_spec.rb
./spec/jobs/subscription_placement_job_spec.rb
./spec/lib/open_food_network/customers_report_spec.rb
./spec/lib/open_food_network/enterprise_fee_applicator_spec.rb
./spec/lib/open_food_network/enterprise_fee_calculator_spec.rb
./spec/lib/open_food_network/order_cycle_management_report_spec.rb
./spec/lib/open_food_network/order_cycle_permissions_spec.rb
./spec/lib/open_food_network/permissions_spec.rb
./spec/lib/open_food_network/products_and_inventory_report_spec.rb
./spec/lib/open_food_network/products_cache_spec.rb
./spec/lib/open_food_network/products_renderer_spec.rb
./spec/lib/open_food_network/scope_variants_to_search_spec.rb
./spec/lib/open_food_network/scope_variant_to_hub_spec.rb
./spec/lib/open_food_network/user_balance_calculator_spec.rb
./spec/models/cart_spec.rb
./spec/models/enterprise_caching_spec.rb
./spec/models/enterprise_fee_spec.rb
./spec/models/enterprise_spec.rb
./spec/models/exchange_spec.rb
./spec/models/order_cycle_spec.rb
./spec/models/producer_property_spec.rb
./spec/models/product_distribution_spec.rb
./spec/models/product_importer_spec.rb
./spec/models/spree/classification_spec.rb
./spec/models/spree/image_spec.rb
./spec/models/spree/line_item_spec.rb
./spec/models/spree/option_type_spec.rb
./spec/models/spree/order_spec.rb
./spec/models/spree/product_spec.rb
./spec/models/spree/property_spec.rb
./spec/models/spree/taxon_spec.rb
./spec/models/spree/variant_spec.rb
./spec/requests/shop_spec.rb
./spec/serializers/admin/for_order_cycle/enterprise_serializer_spec.rb
./spec/serializers/admin/for_order_cycle/supplied_product_serializer_spec.rb
./spec/serializers/spree/product_serializer_spec.rb
./spec/services/order_factory_spec.rb
./spec/services/order_syncer_spec.rb
./spec/services/subscription_form_spec.rb

Here are the non-feature specs with Variant.on_hand failing:

./spec/controllers/admin/proxy_orders_controller_spec.rb
./spec/controllers/admin/subscriptions_controller_spec.rb
./spec/controllers/cart_controller_spec.rb
./spec/controllers/spree/admin/products_controller_spec.rb
./spec/controllers/spree/admin/variants_controller_spec.rb
./spec/controllers/spree/orders_controller_spec.rb
./spec/jobs/subscription_confirm_job_spec.rb
./spec/jobs/subscription_placement_job_spec.rb
./spec/lib/open_food_network/products_renderer_spec.rb
./spec/lib/open_food_network/scope_variant_to_hub_spec.rb
./spec/models/cart_spec.rb
./spec/models/product_distribution_spec.rb
./spec/models/spree/line_item_spec.rb
./spec/services/order_factory_spec.rb
./spec/services/order_syncer_spec.rb
./spec/services/subscription_form_spec.rb