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