Get siteMetadata via GraphQL

It is important that decoupling between node code and browser code.
The 'gatsby-config.js' file must be kept in node code, but it exposed
into browser code because of the Disqus module.
To decouple this, all of the siteMetadata information is served via
GraphQL.
This commit has this change.

src/templates/post-template.jsx

@@ -34,6 +34,8 @@ export const pageQuery = graphql`
           name
           twitter
         }
+        disqusShortname
+        url
       }
     }
     markdownRemark(fields: { slug: { eq: $slug } }) {