Remove the logfile and logSize columns from the database

It's pointless to store these, since Nix knows where the logs are.
Also handle (in fact require) Nix's new log storage scheme.  Also some
cleanups in the build page.

src/lib/Hydra/View/TT.pm

@@ -2,7 +2,16 @@ package Hydra::View::TT;
 
 use strict;
 use base 'Catalyst::View::TT';
+use Hydra::Helper::Nix;
 
-__PACKAGE__->config(TEMPLATE_EXTENSION => '.tt');
+__PACKAGE__->config(
+    TEMPLATE_EXTENSION => '.tt',
+    expose_methods => [qw/log_exists/]);
+
+sub log_exists {
+    my ($self, $c, $drvPath) = @_;
+    my $x = getDrvLogPath($drvPath);
+    return defined $x;
+}
 
 1;