Use app string in lesson question submit button

Right now the submit button shown in the lesson questions uses the string returned by the WebService. We should try to calculate this value ourselves and use app strings.

All the questions have similar code for this:

        if ($hasattempt) {

            $this->add_action_buttons(null, get_string("nextpage", "lesson"));

        } else {

            $this->add_action_buttons(null, get_string("submit", "lesson"));

        }