Allow the customer to decide if to save the payment source

[elia]

Currently. Solidus does not allow to not save credit card information. This may not be ideal in regard to the privacy and legal implications.

1. Verify if/how this is compliant with regulations (e.g. GDPR)
2. Understand which should be the default (use a configuration?)
3. Provide a tentative effort

Once we have understood the impact and effort, then we will prioritize as we see fit or postpone.

[fthobe]

@elia is this still an issue?

[fthobe]

Yes it is. @jarednorman I assume the credit card controller needs a strong touch to do that. It's something we need for obvious reasons here in Europe and I assume the problem here is the difference in abstraction between Braintree and stripe. If you want to give some guidance we can do that next week.

[fthobe]

Related to #288