We have seen flickering of content on the Issue boards pages. By
improving our selectors, we can prevent that flickering.

Looking at app/views/layouts/_page.html.haml, the selectors:

- `.content-wrapper > .alert-wrapper` for breadcrumbs, flashes and
everything
- `#content-body` for the content of a page

seem more reliable than `.container-limited` which isn't set on _all_
pages.