FIX: #138 allow subsite summary fields to be customized
FIX: #138 allow subsite summary fields to be customized

silverstripe/silverstripe-subsites,mikenz/silverstripe-simplesubsites,silverstripe/silverstripe-subsites,mikenz/silverstripe-simplesubsites,mandrew/silverstripe-subsites,mandrew/silverstripe-subsites