Form submission handler for forum_admin_settings().
function forum_admin_settings_submit($form, &$form_state) {
config('forum.settings')
->set('topics.hot_threshold', $form_state['values']['forum_hot_topic'])
->set('topics.page_limit', $form_state['values']['forum_per_page'])
->set('topics.order', $form_state['values']['forum_order'])
->save();
}