Fix Namespace.find_by_pages_host

What does this MR do?

With #28781 (closed) we introduced Namespace.find_by_pages_host.

Currently it behaves incorrectly - if we have namespace foo.io then Namespace.find_by_pages_host("foo.io") will return the namespace which is, this should happen only when looking for foo.io.gitlab.io (or whatever is the configured domain for Pages).

This MR fixes this by making sure we perform the lookup only if the provided host end with the domain configured for Pages.

Screenshots

Does this MR meet the acceptance criteria?

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