Resolve "Fix showing only free namespaces when applying licenses"

What does this MR do?

Fix the User#managed_free_namespaces scope to not rely on the namespaces.plan_id column, since it is not used.

Instead, join with the gitlab_subscriptions and plans tables.

This MR also moves the with_counts(archived: false) method to the SubscriptionsHelper (where it is used in order to prevent n+1 queries when collecting member_counts), because member counts are not needed in the SubscriptionsController, where the scope is also used.

Closes #208894 (closed)