Orphaned branch projects should not be considered buildable

WorkflowJob should consult WorkflowJobProperty for isBuildable, and BranchJobProperty should in turn consult Branch.isBuildable. This should prevent orphaned jobs from being scheduled.

This attempt by stephenconnolly is probably wrong, as it would not affect, for example, TimerTrigger. It is not a permission check which we want, but rather an emulation of disablement.