Support "unbox-startfile" metadata line

[erkyrath]

If present, the unbox link should be of the form https://unbox.ifarchive.org/?url=/if-archive/games/springthing/2017/Ishmael.zip&open=index.html.

[erkyrath]

Unbox supports open=, but when I went to add this, it seemed like not the right solution.

Our normal path is that the "View contents" link goes to an Unbox directory listing, and that listing has a prominent "Open (this game)" button at the top. If "View contents" goes straight to the game, that's inconsistent.

Either (a) open= should show the usual directory listing with the specified file as the prominent button, or (b) we should go back to the original plan where Unbox detects the unbox-startfile metadata line.