Seen anonymously, a private repository's page and its og card say only the title and 'private' #2447

Part 2 of #2349.

Done looks like:

• [ ] Anonymously, the head of a private repository's page (or the page it answers instead) and its og:image card show only a title and the word "private": no description, no owner's activity, no counts.
• [ ] Test: a private repository with a description and an issue. The anonymous head and the card's text leak neither. The owner, signed in, sees the full card.