fix invisible Editor in showcase story

The Editor by default has a height of "100%" in the story, which ends up being 0 pixels because of the preview definition.

But I didn't want to fix it universally in the preview because that could (a) cause other problems in other stories, and (b) wouldn't make obvious that you must specify the height for the Editor. So, I updated the args in the story. While I was in there, I also improved the readability of the Template/container in there.

I did also find an actual problem -- apparently Ace Editor requires some value be set on initial load, and this causes problems if you are e.g. trying to load an editor asynchronously. The solution is to give it a dummy value and then set it to the desired value after load.