when a tag <foo> is selected and only appointments for foo are shown, the "show past appointments" link should refer to http://cal.example.com?tag=foo&past=true rather than http://cal.example.com?past=true
when a tag is selected and only appointments for foo are shown, the "show past appointments" link should refer to http://cal.example.com?tag=foo&past=true rather than http://cal.example.com?past=true