Shipping refunding and credit memos generation

New features:

• refunding order shipment (single-shipment support)
• simple credit memo generation (with credit memo number, order number, total and refunded units)

Improvements:

• refunding all order units with one click
• already refunded units application test
• not being able to refund units from not paid order
• back button added to refunds page
• marking an order as fully refunded when the refunded total is equal to the order total