Support mathjax with GitLab wiki

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/14627

Customer requests that we support the Mathjax library in GitLab's wiki. It doesn't seem hard to do, and someone has a post of how they did it - http://nd.psychstat.org/blog/integrate_mathjax_with_gitlab

It might be a simple win for the wiki users.

cc/ @JobV