Transition missed _config.yml changes to new _config files

With the transition to the new "_config" directory structure, some
of the changes from _config.yml were missed. Integrating all changes
based on _config.yml@9659a45.

* _config/defaults/classic.yml: Add missed blog_index_sidebar,
  page_sidebar, post_sidebar entries; remove duplicate excerpt_link
  entry.
* _config/defaults/disqus.yml: Add missed disqus_developer entry.
* _config/defaults/google_plus.yml: Add missed
  google_plus_image_size entry.
* _config/defaults/jekyll.yml: Add missed include entry; add missed
  markdown=redcarpet and supporting entries.
* _config/defaults/sharing_buttons.yml: Add missed
  respectfully_social entry.
Merge pull request #1001 from tonyduckles/new-configs-fixes

Transition missed _config.yml changes to new _config files