A startup founded by a former social worker is testing a new senior living concept, one that borrows ideas familiar to fans of a certain 1980s sitcom. The idea, from Minneapolis-based Vibrante founder Roxanne Cornell, is this: Buy a house and lease it to four women, age 55 and older. The tenants she's looking for are characterized as active, single women who no longer want to live alone, looking to downsize, and who don't want to live on a senior living campus that can have u