I want a group of people to be able to completely manage a subscription, including managing access to resources within it, except for managing the subscription itself. So (for example) when a new resource group with a storage account is added to the subscription, I want them to be automatically (by inheritance) have all rights to the storage account, including the right to give people roles on that storage account. I just don't want them to be able to give other people roles on the subscription itself, so no adding administrators to the subscription etc.
Is a role (or combination of roles) built-in that I can use for that? Will I have to look into creating a custom role, or is what I'm looking for not possible?