Allow class="noshiftselect" to disable the multiple-select-on-shift-click behavior...

Allow class="noshiftselect" to disable the multiple-select-on-shift-click behavior for checkboxes. This is desired for forms that contain tables with multiple columns of checkboxes (CentralNotice has one of these), where this function works row-wise instead of column-wise. This is confusing and I'm too lazy to implement column-wise shift-select, so I'm just disabling shift-select altogether in CentralNotice (see followup commit).