Extend online view of HTML artifacts to private projects

Description

Issue gitlab-ce#34102 allows to view HTML artifacts in a browser window instead of download them locally, but it is limited to public projects only.

Since most users need this also for private projects, a proper support to that is the scope of this issue.

Proposal

Extend online view of HTML artifacts to private projects, providing proper authentication to rescrict artifacts access.

Documentation blurb

Use cases

Using GitLab to create test/coverage reports in HTML format, that must be accessible only by authorized users.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml