Fixes race condition for GFM rendering on issuable description

What does this MR do and why?

Solves #432967 (closed)

Sometimes mermaid diagrams are not getting rendered on issuable (reproduced with incidents). Based on this comment it's due to a race condition:

it seems like it's a race condition. We try to render mermaid before the description is rendered to DOM, which fails. Fortunately, the fix is as easy as adding a nextTick in the right place, so it would be a one-line fix.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Add an incident and add a mermaid diagram to the description. You can use the following as an example and add it inside a mermaid codeblock:

   gantt
       title A Gantt Diagram
       dateFormat  YYYY-MM-DD
       section Section
       A task           :a1, 2014-01-01, 30d
       Another task     :after a1  , 20d
       section Another
       Task in sec      :2014-01-12  , 12d
       another task      : 24d

2. Because it's a race condition it might or might not render properly. With these changes, it should always render.