Notify Admin users when a new user signup

Similar to the PreSignup trigger, Cognito has support for the PostConfirmation trigger.

This trigger is invoked after a user is confirmed, allowing us to send custom messages or to add custom logic, for example for analytics.

In our case, we will send out an information email to all of the Admin Users (those who are part of admins group) that a new user has signed up for the ServerlessDocs access.