Use UserNamespacePolicy for user namespaces

What does this MR do and why?

We're converting to using Namespaces::UserNamespace.

This MR converts NamespacePolicy into Namespaces::UserNamespacePolicy

FYI, GroupPolicy does not inherit from NamespacePolicy, as one might expect. They both inherited from BasePolicy. This means we can make Namespaces::UserNamespacePolicy inherit from BasePolicy, and have NamespacePolicy inherit from Namespaces::UserNamespacePolicy.

MR acceptance checklist

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Related to #341124 (closed)