Support multiple keys with the same AWS account

[mbernst]

Right now our primary user identifier is the underlying AWS account. But multiple people can have different independent keys for the same account. We may want to rearchitect to create our own primary key and use that, allowing the use of multiple IAM accounts from the same AWS. Behavior right now is that if the second user signs up with their keys, it overrides the keys and email address of the prior person.