Create admin account step in Setup dies with an ungraceful DuplicateEntityException

When customers upgrade confluence and point the new version back to their existing DB, creating a new admin account is irrelevant.

When the admin attempts to re-add themselves for the sake of completing this step, the Setup process dies with a 500 error page. We need to:

1. allow them to skip this step OR
2. check for existing username for a more graceful handling of this error