dcache-webadmin: fix regression affecting checkall on alarms table

Recent update to Wicket 1.6 broke the javascript for the "check all" checkboxes on the Alarms table.

This patch restores it.

Testing:

Checked by clicking.

Target: master
Patch: http://rb.dcache.org/r/6084/
Request: 2.7
Acked-by: Gerd
Require-notes: no
Require-book: no
Bug: http://rt.dcache.org/Ticket/Display.html?id=8022