Update to support inverse value for refunds

Due to a bug in the Fat Zebra sandbox the refund amount was being sent through as a negative amount, whereas the Live gateway would return a positive amount. This means that in a live website the refund was being counted as a positive amount and skewing the order transactions.

This change checks for the '-R-' in the ID (which indicates a refund record) and flips the value (e.g. multiply by -1 to make it a negative) before applying it to the transaction.
Added missing param for call to commerce_fatzebra_response_process

The call to this method is expecting 3 params and the 3rd param is used. Adding this prevents a warning from being raised during refund etc.
Added 'refundable amount' to form

Improve the UI by showing the merchant how much remains to be refunded.
Merge remote-tracking branch 'origin/patch-1' into 7.x-1.x