Fix issue with JSON card data not rendered in the preview pane

[kolkheang]

This PR corrects issue #121.

Code Mirror CSS and global CSS need this specific configuration to work properly.
Applying the original logic with this PR

[kolkheang]

@dennispatterson & @jmandel
Please review

[dennispatterson]

Thanks, @kolkheang! This gets the code showing up for editing, but don't think the CSS is quite right yet.

Original header:

With these changes:

Also, the code in the editor is missing brackets and colons (they're being rendered in white):

[kolkheang]

Thanks for the feedback, @dennispatterson. Let me try to revert back to the old version of codemirror.

[kolkheang]

@barabo I don't have priv to add you as another reviewer. But, if you have some time, I wouldn't mind if you could also take a look at the style issues.

[kolkheang]

I track down the css issue to this https://github.com/cds-hooks/sandbox/blob/master/src/components/CardDemo/card-demo.css#L53 line which makes brackets in the code appear white. I'll adjust that so that everything is visible.

I will take a look at the buttons' color issue next.

[dennispatterson]

Thanks, @kolkheang! That seems to fix the brackets/text. Any idea what's going on with the header?