Adding a responsive secondary footer menu

For others following this topic that want to do the same thing, here are the important parts:

1. https://gist.github.com/generatepress/bc55ba4d42f7cc26e91c - adding PHP: https://generatepress.com/knowledgebase/adding-php-functions/

2. https://gist.github.com/generatepress/56e84848ee0c6ab6bdb4 - add JS: https://generatepress.com/forums/topic/adding-a-responsive-secondary-footer-menu/#post-134512

3. https://gist.github.com/generatepress/4860e09d80c73b18bf73 - adding CSS: https://generatepress.com/knowledgebase/adding-css/

4. https://generatepress.com/forums/topic/adding-a-responsive-secondary-footer-menu/page/2/#post-136799

5. https://generatepress.com/forums/topic/adding-a-responsive-secondary-footer-menu/page/3/#post-138559

6. https://generatepress.com/forums/topic/adding-a-responsive-secondary-footer-menu/page/4/#post-144719