Removes `_plugins/google-scholar-citations.rb` and
`_plugins/inspirehep-citations.rb` and switches to using `al_citations`
gem instead.

Removes `_plugins/external_posts.rb` and switches to using
`al_ext_posts` gem.

…2978)

I agree that it is a good idea to unbundle some of the plugins from
`al-folio`, specially since they can be useful to more jekyll users
overall. I have unbundled 2 plugins and created a gem for each one of
them, so they can be easily integrated in any jekyll site. This is the
first one of them.