webadmin: Implement default sort order for CheckboxColumn

Similarly to what was done with TextColumnWithTooltip, it makes sense
to implement some default sorting behavior for CheckboxColumn.

Change-Id: I7c70dd4c052ba21660f16ae223eb00ccf5d5edba
Bug-Url: https://bugzilla.redhat.com/1120054
Signed-off-by: Lior Vernia <lvernia@redhat.com>