Fix open graph title/description on home page

Previously, this was showing a title of "Topics in ~" in embeds and
such, due to using the same title/description as group topic listings,
but without a group name.

tildes/tildes/templates/home.jinja2

@@ -7,6 +7,11 @@
 
 {% block title_full %}Tildes{% endblock %}
 
+{% block meta_og_tags %}
+  <meta property="og:title" content="Tildes">
+  <meta property="og:description" content="Tildes is a non-profit community site with no advertising or investors. It respects its users and their privacy, and prioritizes high-quality content and discussions.">
+{% endblock %}
+
 {% block header_context_link %}{% endblock %}
 
 {% block content %}