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.

src/lib/Hydra/Controller/Root.pm

@@ -283,7 +283,7 @@ sub narinfo :LocalRegex('^([a-z0-9]+).narinfo$') :Args(0) {
     my $path = queryPathFromHashPart($hash);
 
     if (!$path) {
-	$c->response->status(404);
+        $c->response->status(404);
         $c->response->content_type('text/plain');
         $c->stash->{plain}->{data} = "does not exist\n";
         $c->forward('Hydra::View::Plain');