Signup form account setup stage

[ghost]

Sometimes pre-existing accounts/users who are acting as collaborators eventually want to add another client or subscription under their own account. When they go to atomic-beta.pagely.com/signup and enter their email + pw during the second step, they wind up in a loop since they're trying to register a pre-existing address already in our system.

This request is to send those users a notification to the effect of "The email you entered is already registered with an account. If you're looking to add a subscription or make any changes, please login to Atomic."

This way, there isn't any confusion around why their info isn't processing successfully.

(This is low-priority but I wanted to submit formally to keep on our radar)