Support a 'page top' container

Should we support a generic container for placing some content at the top of the page in a way that SiteNav and PageNav play along? i.e., SiteNav and PageNav should appear below this container. 

e.g., 
```html
<pagetop sticky flush>
... content
</pagetop>
```

Example user case: if the user wants to use Bootstrap navigation menus instead of our own `navbar`