Updates $is_mobile()

I have changed the `is_mobile()` function so that it returns a boolean.

    function is_mobile() {
      $userAgent = strtolower($_SERVER['HTTP_USER_AGENT']);
      return !!strpos($userAgent, 'mobile');
    }

This renders this if/else block unnecessary:

    if(!is_mobile()){
      $is_mobile = "FALSE";
    } else {
      $is_mobile = "TRUE";
    }

I have removed all occurrences of this and replaced it with `$is_mobile = is_mobile();`.