They’re the raddest bike-sharers around. Daredevil Lower East Side cyclists are turning clunky, notoriously heavy CitiBikes into their personal stunt bikes, pulling off gravity-defying stunts with wheelies across alleys and jumps off stairs, then posting videos of the free-wheeling antics to an Instagram account called “Citibikeboyz.” The ‘boyz’ behind the account say they actually feel...