[api] Specify stock_items key in variants/index so key is 'stock_items' rather than 'spree_stock_items'
make sure the options are actually created.

Fixes #4271
Fixes #4273

Conflicts:
	backend/app/assets/javascripts/spree/backend/stock_transfer.js.coffee

Conflicts:
	backend/app/assets/javascripts/admin/stock_transfer.js.coffee