MermaidJS broken since GitLab EE 10.4.0-rc7

Summary

MermaidJS markdown is broken in GitLab issues.

Steps to reproduce

Graph LR
    Ping <--> Pong

Example Project

This issue

What is the current bug behavior?

Graph is not showing, entry keeps blank

What is the expected correct behavior?

Graph to be shown

Output of checks..

This bug happens on GitLab.com