Add user에 isAdmin추가

src/main/java/com/example/springsecuritystudy/user/User.java

@@ -39,6 +39,10 @@ public class User implements UserDetails {
 		return Collections.singleton((GrantedAuthority) () -> authority);
 	}
 
+	public Boolean isAdmin() {
+		return authority.equals("ROLE_ADMIN");
+	}
+
 	@Override
 	public boolean isAccountNonExpired() {
 		return true;