Taxon edit form layout tidyup

Fixes #3042
Consolidate spree/order attributes for coupon_code
Add backorderable default config to StockLocation [Fixes #3045]

Stock items backorderable values will be set according to this config
every time either a variant or a stock location is created
Add propagate variants config to StockLocation [Fixes #3050]

Every time either a new stock location or new variant is created the
stock_location##propagate_all_variants value will be checked to decide
whether new stock items should be created for the new stock location or
variant
Respect CanCan.
Merge git://github.com/spree/spree