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

Fixes #3733
remove unnecessary semicolon in screen.css.scss

Fixes #3729
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