Mark pay run warnings as "completed" or "acknowledged"

It would clients and processors greater peace of mind to be able to mark off warnings once they had been actioned, in much the same way as we mark processing notes. This would speed up our checking process, as we would know at a glance which warnings we had already investigated and would flag any new warnings that had arisen as a result of subsequent changes. Once all warnings had been checked off, the red bar in the pay run screen could then change to green, or show a count of the number of warnings outstanding.