Move blog to separate repository

From #1265 (comment 42447351), here's the status:

https://gitlab.com/gitlab-com/blog contains a stripped down (and slightly stale) version of www-gitlab-com repository with mostly just the blog. Here's what we need to do:

• Make all URLs linked to use the /blog prefix.

  What I've tried: middleman-blog has a prefix configuration, and Middleman has a :http_prefix configuration value. It seems that if we set the prefix in the middleman-blog, we have to move everything in sources/ into the sources/blog directory. middleman-blog then generates everything in the public/blog directory.

  However, all helper functions (e.g. javascript_include_tag) used by Middleman won't know to use the /blog prefix, and hence JavaScript assets will be pointing to /javascripts/, for example. If you set http_prefix, the JavaScript paths correctly use /blog/javascripts, but you end up getting a double /blog/blog/2017 for blog entries.

  We might be able to fix this by setting :css_dir, :js_dir, and :images_dir independently for Middleman: http://willschenk.com/building-sites-with-middleman/

• Add a redirection rule in the NGINX about.gitlab.com configuration to migrate /20xx/ -> /blog/20xx.

• rsync or copy new blog posts/images from www-gitlab-com into the new repository.

• Fix the .gitlab-ci.yml on both repositories to deploy to different internal paths (e.g. blog-public and www-public).

• Adjust the about.gitlab.com NGINX configuration to point /blog to blog-public, and everything else to www-public dir.

• Adjust the about.gitlab.com configuration to work with Review Apps.

• Optional: Run git filter-branch or some sort to rewrite history to trim down the repository size.

I could use some help with this. @axil, @rspeicher, perhaps you could lend a hand?