BUG Fixing regression from silverstripe/sapphire/67d1327

Status flag classes didn't pick up styling because the flag was changed to prefix "status-".

https://github.com/silverstripe/silverstripe-framework/commit/67d1327b90187b25877af05fbb5046176153ef92 introduced a change in CSS classes, but SiteTree is still outputting the old style one.

This fixes it to prefix all the flag classes with "status-" so the styling is picked up correctly.