Onboarding: Enhance super admin assignments

[Ayush8923]

Is your feature request related to a problem?
There are issues in the onboarding flow identified during testing. These problems hinder the proper assignment and mapping of super admins.

Describe the solution you'd like
Enhancements needed for onboarding:

1. Don't allow to assigning existing users as super admins if they are part of another project or organization.
2. Ensure the super admin is mapped in the user_project table during organization onboarding, in addition to being added to the user table.

Original issue

Describe the current behavior
Currently, there are a few issues in the onboarding flow that came up during testing and need to be addressed.

Describe the enhancement you'd like

1. When onboarding an organization, we need to assign a super admin. If the user already exists under another project or organization, they currently cannot be assigned as a super admin.
2. During organization onboarding, the super admin user is currently being added only to the user table. The same user also needs to be mapped in the user_project table.