Draft: Improve OpenGraph/Twittercard previews, fixes #5

• Change og:type for articles
• Change image shown in preview based on if an image tag exists
• Use article:tag for tag words associated with an article

source/layout/base_head.xml

@@ -3,7 +3,6 @@
   <meta http-equiv="x-ua-compatible" content="ie=edge"/>
   <meta name="viewport"
             content="width=device-width, initial-scale=1, shrink-to-fit=no, maximum-scale=5.0"/>
-  <meta content="website" property="og:type"/>
   <meta content="summary" property="twitter:card"/>
   <meta content="/images/ogp.png" property="og:image"/>
   <meta content="724" property="og:image:width"/>