Add Open Graph meta tags

app/views/layouts/_head.html.haml

 - page_title "GitLab"
-%head
+%head{prefix: "og: http://ogp.me/ns#"}
   %meta{charset: "utf-8"}
   %meta{'http-equiv' => 'X-UA-Compatible', content: 'IE=edge'}
   %meta{content: "GitLab Community Edition", name: "description"}
   %meta{name: 'referrer', content: 'origin-when-cross-origin'}
 
+  -# Open Graph - http://ogp.me/
+  %meta{property: 'og:title', content: page_title}
+  %meta{property: 'og:url',   content: url_for(only_path: false)}
+
   %title= page_title
 
   = favicon_link_tag 'favicon.ico'