Expose pages unique host instead of unique url

What does this MR do and why?

While working on Redirect to unique domain (gitlab-pages!856 - merged) we noticed that it'd be easier to building the redirect URL from the uniqueHost instead of the uniqueURL.

• Reasoning: gitlab-pages!856 (comment 1333322286)
• Related to: Redirect namespace domain to unique domain (gitlab-pages#831 - closed)

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