Add Disqus comments to blog posts

Per instructions at
http://spring-io.disqus.com/admin/settings/universalcode, three 'sites'
have been set up at Disqus under the 'saganops' account:

 - spring-io (production)
 - spring-io-staging
 - spring-io-localhost

Depending on the active Spring profile (see changes to application.yml),
one of these names will be exposed to the frontend via BlogController as
'disqusShortname'. See changes to _full_post.html to see where this is
assigned to disqus_shortname.

Comment threads are correlated with individual posts using
disqus_identifier, which is assigned the (unchanging) database id of the
post being rendered.
Make fields private where possible
Merge pull request #334 from spring-io/disqus

Add Disqus comments to blog posts