Do not generate a new identifier for a payment when it is re-saved

Fixes #3733
[backend] Move 'Taxonomies' configuration out from under 'Configuration' to 'Products' subheading
[backend] Make checks for order totals more specific in specs

Probably fixes strange build failures like: http://ci.spreecommerce.com/viewLog.html?buildId=22440&buildTypeId=bt6&tab=buildLog
Product#stock_items should return stock_items for the master variant too

Fixes #3737