E2e rx disk size

Closes https://github.com/rackerlabs/encore-ui/issues/322
Now that I think about it, I don't think we should be tying in to the `.tab-content` selector. If we do, any HTML update we make to the demo page would require updating all of our midway tests just to get the selector in a good place. 

Adding a `component-demo` class [around the 'Demo' tab](https://github.com/rackerlabs/encore-ui/blob/eddd05790c2b00235695378519d4c3a4cbb73126/demo/component-template.html#L13) resolve the issue:

```
<tab heading="Demo">
    <div rx-compile="component.docs.html" class="component-demo"></div>
</tab>
```