Transition missed _config.yml changes to new _config files

With the transition to the new "_config" directory structure, some of the intermediate changes from _config.yml were missed. Integrating all missed changes based on _config.yml @9659a45 (predecessor to 6ce4d89). 

* _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.