Administrate/clean incomplete/abandoned payments. Avoid over-payments.

[jedateach]

Some payments fail, so they show up as "Authorized" or "Created". Payments should not remain in these statuses. Sometimes these payments may have actually succeeded in the gateway.

There are a few ways to clean these up:

• Administrators should be able to run the 'completePayment' function so to do a lookup that either voids the payment, or completes it.
• Administrators should be able to complete/void manual payments. Replaces Allow admins to complete manual payments #51
• Automatic task can tidy up payments that are older than X period.
• When a payment is re-attempted on the same Payable object, it could check if incomplete payments were successful.

[bummzack]

Completing payments is possible with 2.0 and the omnipay-ui module
Deleting pending payments upon re-attempting is a tricky one… especially with gateways that use async notifications to notify of payment completion. Maybe better not do that…

Closing this and adding a follow up issue: #135