Declarative jobsets: move event handling to a plugin

Declarative jobsets were sort of tucked in to the event hanlder
itself. It turned out that it could have been implemented as a
plugin without much trouble.

src/script/hydra-notify

@@ -49,12 +49,6 @@ sub buildStarted {
 sub buildFinished {
     my ($build, @deps) = @_;
 
-    my $project = $build->project;
-    my $jobsetName = $build->get_column('jobset');
-    if (length($project->declfile) && $jobsetName eq ".jobsets" && $build->iscurrent) {
-        handleDeclarativeJobsetBuild($db, $project, $build);
-    }
-
     my @dependents;
     foreach my $id (@deps) {
         my $dep = $db->resultset('Builds')->find($id)