Add one-shot jobsets

There are jobsets that are evaluated only once, that is, after they've
been evaluated, they're disabled automatically.  This is primarily
useful for doing releases: for instance, doing an evaluation with
"officialRelease" set to "true" should be done only once.

src/lib/Hydra/Helper/Nix.pm

@@ -540,7 +540,7 @@ sub grab {
 sub getTotalShares {
     my ($db) = @_;
     return $db->resultset('Jobsets')->search(
-        { 'project.enabled' => 1, 'me.enabled' => 1 },
+        { 'project.enabled' => 1, 'me.enabled' => { '!=' => 0 } },
         { join => 'project', select => { sum => 'schedulingshares' }, as => 'sum' })->single->get_column('sum');
 }