fix: check for existing superuser

[maranmaran]

Fixes #563 and #537

Motivation

When creating superuser, currently there's no guard to protect against already created superuser. If you seed superuser more then one times, it breaks login for that particular user as the request throws InternalServerError on login

Modifications

In the same style of other guards within UsersController I've added query to repository to get entity by name for the incoming user payload. If the entity exists we return validation error.

Verifying this change

• [x] Make sure that the change passes the ./gradlew build checks.

[lhotari]

Thanks for the contribution @maranmaran