More intuitive UI to disable #resizable on textareas

Even though I have made that way, I've been thinking that this might not be the best UI. Maybe we need to change the checkbox to "Disable resizable of this textarea" acting on TRUE instead of "Resizable textarea" acting on FALSE. This would also help in solving another issue (#2304557: Disable resizable from summary on fields of type text_with_summary) as a new option - maybe called "Disable resizable on summary too" - could appear when this was checked.