Enable previewing private artifacts

Merged Tuomo Ala-Vannesluoma requested to merge tuomoa/gitlab:preview_private_artifacts into master

What does this MR do?

This MR enables the external (pages) preview link for also private artifacts when pages access control is enabled.

This depends on the actual gitlab-pages implementation gitlab-pages!134 (merged). The technical details are explained on the gitlab-pages MR.

This merge request is moved from the old gitlab-foss to here. Old merge request is gitlab-foss!25567 (closed).

What are the relevant issue numbers?

Implements #14897 (closed) gitlab-foss#10982 (moved)

Does this MR meet the acceptance criteria?

Edited by Tuomo Ala-Vannesluoma