A forge preview's Looks always include the seeded pull page, repository head, home with pins and /flow, at desk and phone width #8974

Replaces #8865, whose seeding shipped. Done when:

• [x] The list of pages a forge preview takes Looks of includes these four, as the owner, at desk and phone width, from one named list in code.
• [x] A test asserts the list holds all four at both widths, so dropping one fails the check.
• [x] The pull links one preview's Looks showing them.

Files: the looks/preview seeding module, a test.

Reviewed pull #9000 at its head: ticked The list of pages a forge preview takes Looks of includes these four, as the owner, at desk and phone width, from one named list in code.; A test asserts the list holds all four at both widths, so dropping one fails the check.; The pull links one preview's Looks showing them..