Add a ‘latest-finished’ action

It redirects to the latest successful build from a finished
evaluation.  This is mostly useful for the Nixpkgs/NixOS mirroring
script, which need the latest finished evaluation in which some
aggregate job (such as ‘tested’ in NixOS) succeeded.
Add some more fields to the JSON output of /build
Fix some XML wellformedness issues
Add a /eval action to /build to go from a build to the latest finished eval