Add a link to the raw log.

[iwanders]

I could not find a link to download the raw log in the log view, so for example on this (randomly picked) page, I would expect a download link; https://hydra.nixos.org/build/175606766/nixlog/2125

The raw log is accessible and served by Hydra, by adding /raw to the URL, we obtain: https://hydra.nixos.org/build/175606766/nixlog/2125/raw which in turn redirects to; https://hydra.nixos.org/log/bs93xva3z3pkig6f5gaxj6pgmmpmx59y-squashfs.img.drv

I think there's a lot of value in providing a link to the raw log, such that it is convenient for users to download this, for example if they want to grep in it locally.

This PR adds a link to the log overview page that links to the raw log, it looks like this:

The logic with the ternary is copy pasted from the tail section in the same file.

fyi @mikepurvis