Add meta data for sharing

[michaelsnook]

This should work for both the public.html template and for base.html. We should be able to see a good title and description when sharing on popular platforms like Facebook and WhatsApp.

Individual cards might show a different description than pages like "add a card" and "add a deck" so at least for base.html, and possibly for both top-level templates, we should use a {% block sharemeta %}.