Allow multi-select of acked alerts

Once an alert has been acked, it no longer has a checkbox, so you can no longer multi-select that alert.

I had 28 alerts this morning. I selected all and Acked. I later found that all 28 were related to the same problem and I wanted to put them in SDT or at least add a note. Since they are already acked, I can't select multiple alerts, and therefore can't add a note or SDT to multiple alerts at once. Would be nice if the checkboxes didn't get replaced by a graphic after they are acked.