Render comments via platform-agnostic inclusion

[ashmaroli]

• Move markup to render disqus comments into _includes/comments.html. Rather than renaming the include_file, the following changes have also been made:
  • move JEKYLL_ENV=production and comments: false front matter checks up into post layout.
  • move check for site.disqus.shortname into _includes/comments.html.
  • this.page.identifier javascript variable is set to {{ page.id }}.
• Render a Comments have been disabled for this post. message in posts with comments: false in front matter.

Rationale

A generic inclusion / include_file name would mean a user could opt to another comments-provider-platform by editing the generic include_file rather than editing the post layout as well as introducing a new include_file at user's source-dir.

[ashmaroli]

@jekyllbot: merge +major