Clarify that Organizations will be as thin as possible

What does this MR do and why?

This MR proposes that we do not shift any functionality to the Organization that is not absolutely necessary to achieve the Cells 1.0 scope. In this case, that means that we leave much of the authentication and sign-in functionality for SaaS to remain at the top-level group.

The only users who would need to be specifically invited to the Organization and granted a role, are those who would manage the Organization itself.

It is likely that we will need to do further exploration and validation of this, but this MR sets the intended direction and is a two-way door if we need to revisit.

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