Fix 404 for pipelines

What does this MR do?

Allow to access pipelines of public project by logged guest.

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17885

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together