Fix logo size

Also, always include the text "Hydra" in the navbar.
Use hashFile instead of nix-hash

Calling a shell command is a security hole if $path contains special
characters (e.g. "${foo}").  Observed in
http://hydra.nixos.org/build/4041321.
Disallow build products that are symlinks

Otherwise you can do

  ln -s /etc/passwd $out/foo
  echo "file misc $out/foo" >> $out/nix-support/hydra-build-products

and get Hydra to serve its /etc/passwd file.