Add Disqus to docs.gitlab.com
We are trying to implement Discourse as the commenting system to docs.gitlab.com, but we faced an inconsistent error: #132 (closed).
This MR replaces Discourse with Disqus:
Review Apps: http://implement-disqus-comments.178.62.207.141.xip.io/ee/user/project/index.html
Closes #133 (closed)
cc/ @axil