Allow build to be bumped to the front of the queue via the web interface

Builds now have a "Bump up" action. This will cause the queue runner
to prioritise the steps of the build above all other steps.

src/sql/upgrade-40.sql

@@ -0,0 +1,5 @@
+alter table Builds add column globalPriority integer not null default 0;
+
+create function notifyBuildBumped() returns trigger as 'begin notify builds_bumped; return null; end;' language plpgsql;
+create trigger BuildBumped after update on Builds for each row
+  when (old.globalPriority != new.globalPriority) execute procedure notifyBuildBumped();