Add some basic Open Graph meta tags

This should allow for some basic embeds to be displayed in
Slack/Discord/Twitter/etc. when people link to Tildes urls. This should
definitely be enhanced by adding things like descriptions to pages where
that's relevant, but it's a reasonable start.

tildes/tildes/templates/base.jinja2

@@ -11,6 +11,13 @@
   <meta name="intercoolerjs:use-data-prefix" content="true">
   <meta name="csrftoken" content="{{ get_csrf_token() }}">
 
+  <meta property="og:image" content="/images/tildes-logo-144x144.png">
+  <meta property="og:site_name" content="Tildes">
+  <meta property="og:type" content="website">
+  {% block meta_og_tags %}
+  <meta property="og:title" content="{{ self.title() }}">
+  {% endblock %}
+
   {# Hardcoding each option isn't great, but I don't know a better method #}
   {% if request.cookies.get('theme', '') == 'dark' %}
   <meta name="theme-color" content="#073642">