UVALDE, Texas -- Salvador Ramos told girls he would rape them, showed off a rifle he bought, and threatened to shoot up schools in livestreams on the social media app Yubo, according to several users who witnessed the threats in recent weeks.But those users -- all teens -- told CNN that they didn't take him seriously until they saw the news that Ramos had gunned down 19 children and two adults at an elementary school in Uvalde, Texas, this week.Three users said they witnessed Ramos threaten to commit sexual violence or carry out school shootings on Yubo, an app that...