[Cloud] Default `admin` user

[cal-overflow]

A default admin user should be created when the system is first deployed. The pin can be something simple like password or admin (similar to how a lot of internet routers are set up).

The system user can go in and update the user's name and pin.

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This can likely be implemented using a AWS::CloudFormation::CustomResource that performs a putItem on the user dynamodb table.

[cal-overflow]

Update docs with information regarding this default user