Make it explicit when logs have been deleted

[Ekleog]

I was trying to look at the full log of NixOS/nixpkgs#34971, and came upon https://logs.nix.ci/?key=nixos/nixpkgs.34971&attempt_id=4c2c7a7f-b031-4f55-bcc2-b3faf558089e. The build having been completed for 18 days (so more than 7 days), its full log has been deleted since then.

I think it would avoid “wtf‽” reactions to have a message like “The logs have expired in order to save space to new logs. Sorry!” (in huge blinking red?) somewhere on the page, so that people don't wonder why there is no log available while the partial log stored on github clearly shows there was data there.

Anyway, thanks for your work!

[samueldr]

[20:20:29] <gchristensen> well it is challenging because the viewer has no way of knowing if it was deleted or not
[20:20:38] <gchristensen> it just ... isn't there, know what I mean?
[20:20:42] <samueldr> gchristensen: when you delete, you could leave an IOU :)
[20:20:44] <gchristensen> but samueldr and I can sort something out

https://logs.nix.samueldr.com/nixos-dev/2018-03-14#1010135;