Add slug history to product. Fixes #5014
Finish replacing old refund system:

- Stop requiring negative adjustments in order to perform refunds
- Add new admin pages to manage refunds
- Remove Payment#credit! (and associated controller actions) and instead use refund objects that are linked to payments
- Update Payment#cancel so that it no longer falls back to a full refund
- Refactor some code in the Refund model

Fixes #5012