Add basic Twitter card

Twitter should fall back to Open Graph meta tags [1], but this doesn't seem
to work properly. This commit adds a basic "Summary" Twitter card using the same
title, image, and description as the Open Graph tags.

[1] https://developer.twitter.com/en/docs/tweets/optimize-with-cards/guides/getting-started#opengraph

static-site/src/components/SEO/SEO.jsx

@@ -88,6 +88,12 @@ class SEO extends Component {
           property="fb:app_id"
           content={config.siteFBAppID ? config.siteFBAppID : ""}
         />
+
+        {/* Twitter Card tags */}
+        <meta name="twitter:card" content="summary" />
+        <meta name="twitter:title" content={title} />
+        <meta name="twitter:description" content={description} />
+        <meta name="twitter:image" content={image} />
       </Helmet>
     );
   }