Add free answer question feature when importing  - refs BT#8660

Add free answer question feature when importing  - refs BT#8660
This form is equivalent to:
    $type = FREE_ANSWER;
    if ($correct == 1) {
        $type = UNIQUE_ANSWER;
    } elseif ($correct > 1) [
        $type = MULTIPLE_ANSWER;
    }

However, this latter version is preferred because:
* it has one assignation less (at least in a fraction of the cases)
* it sets a default value that is already useful, independently of the if/elseif/else condition, which is good to ensure that any further modification of the condition will not end up in a situation where $type remains == ''

I'm accepting this PR anyway, but please consider this for your next PRs :-)