Add Twitter card (2factorauth#4188)

_config.yml

@@ -9,9 +9,10 @@ description:
 url: https://twofactorauth.org
 repo: https://github.com/2factorauth/twofactorauth
 img: img/logo.png
+twitter: 2faorg
+social_image: https://repository-images.githubusercontent.com/17724730/63495880-b408-11e9-8cc4-03451aa4cc14
 sass:
     style: compressed
-
 exclude:
     - CNAME
     - CONTRIBUTING.md
@@ -23,3 +24,4 @@ exclude:
     - verify.rb
     - vendor
     - websites_schema.yml
+    - api/v2/README.md

_layouts/default.html

@@ -14,6 +14,13 @@
   <meta property="og:url" content="{{ page.url | prepend: site.url }}">
   <meta property="og:description" content="{{ page.description | default: site.description }}">
 
+  <meta property="twitter:card" content="summary_large_image">
+  <meta property="twitter:title" content="{{ page.title | default: site.title }}">
+  <meta property="twitter:creator" content="{{ site.author }}">
+  <meta property="twitter:site" content="{{ site.twitter }}">
+  <meta property="twitter:image" content="{{ site.social_image }}">
+  <meta property="twitter:description" content="{{ page.description | default: site.description }}">
+
   <title>{{ page.title | default: site.title }}</title>
 
   <!-- Google Fonts should not use SRI hashes as the CSS served dynamically changes depending on browser -->