Extract footer to separate partial twig

[esocode]

Currently the site footer is contained in base.html.twig. That makes it difficult to modify the footer in themes that inherit from Quark because it would either require to copy and modify base.html.twig, thus decoupling it from future Quark updates, or to copy and modify page templates like default.html.twig which would require the replication of code in all pages.

This could be avoided if Quark imported it's default footer twig with {% include 'partials/footer.html.twig' %} which could then be overridden by inheriting themes.

[rhukster]

Done for next release