Allow customers to manage payment methods even if they don't have any subscription

Problem

Due to one of our changes some customers (resellers, to be more specific) ask why they can't manage payment methods.

@annapiaseczna wrote in Slack:

Some resellers would ask to pay via credit card - that means that a customer portal account is created for them where they can add their CC information that syncs into their Zuora account. These accounts don't have subscriptions tied to them as the subscriptions live on the end-user accounts in Zuora. There aren't many requests from resellers to pay via CC so a workaround here would be to get the CC information over the phone. I wanted to understand what the rationale behind the change was to see the overall impact. It certainly makes more sense to not have the banner visible to not confuse customers rather than have the Payment Method tab visible for a small number of resellers that pay via CC. Unless someone can think of a simple way to have it both...

More context in Slack thread