Support notifications for expiring payment methods

[maxrice]

For gateways that support tokenization, we're storing the expiration date for each payment method (if applicable). It would be nice to automatically email customers when their payment method is close to expiring, and provide an easy way for them to update it (especially with the add new payment method flow). This would need to be an admin-enabled option.

Originally requested in ZD ticket #316568

[justinstern]

I smell a new plugin idea coming on!

[ianbmacdonald]

This is a great idea! It might make sense to expose the payment method switch flow too if possible.. i.e. ditch that obsolete credit card for bitcoin.