Allow session evaluations end time to be configable.

Currently, we hard code the end of session evaluations to the following:

    $future = date_create(date('Y-m-d H:i:s', $conference->field_dates[LANGUAGE_NONE][0]['value2']), new DateTimeZone($conference->field_dates[LANGUAGE_NONE][0]['timezone']));
    date_add($future, date_interval_create_from_date_string('2 weeks'));
    if ($current_conferece_time > $future) {
      return;
    }

It'd be nice if there was a configuration option to make this settable per site.