Billing: implement a Billing Manager permissions option

Customer reports: “Only the person that opens the account right now can enter billing information. Even if you’re an admin, you cannot enter billing information for what I’ve seen. Is there a way to solve this in a future product feature?”

While best practice is to restrict the ability to edit billing info in a multi-user SaaS product, the ability to assign this role is desireable and will support the use case where a user signs up with a free account, but needs someone else to input billing info to upgrade. Will also support the ability for users to transition off the team and to have multiple billing managers for backup.