Fix pages unique domain redirect

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Fix pages unique domain redirect

For root domain urls, like namespace.gitlab.io, the unique pages redirect was not working.

The root cause was that the prefix (https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/pages/lookup_path.rb#L48) was wrongly built for unique domain enabled projects since the Project#pages_url was returning the unique domain url, making its comparison with the root domain url never match.

Related to: gitlab-pages#1058 (closed) Changelog: fixed

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