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

Fixes #3733
2 Migrations to ease in the upgrade from 1.3 -> 2.0 * Assign a shipping category to existing products & shipping methods if they don't have one * Migrate old shipping calculators to the new ones and preserve their preferences

Fixes #3605
Fixes #3660
[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