Use Organization of Namespace for Organization for Placeholder User namespace

What does this MR do and why?

In !155732 (merged), we added validation for presence of an Organization on Namespaces. Since this MR was already quite big and impactful, most 'Create Namespace' code paths are wrapped in a Namespace.with_disabled_organization_validation method, which caused the validation to be skipped.

The Gitlab::Import::PlaceholderUserCreator class had this validation still disabled because the specs failed: the assigned organization did not exist

This MR has two changes:

• Use the Organization of the Namespace for the user personal namespace.
• Enable validation of presence of Organization on the user personal namespace

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Related to #411832 (closed)