Make `/archive` a placeholder instead of 404'ing.

[Rémi Audebert]

https://prologin.org/archive/ returns 404 - Not Found, but the archive module should be available in the near future.

We should make this page return a placeholder with a friendly message such as: "Prologin archives are not available right now. Sorry for the inconvenience."

[Antoine Pietri]

Obsolète