Components: Deprecate VerticalNavItem, move functionality to existing or new component

[hoverduck]

Now that <Card /> wraps itself in an anchor and includes a chevron if passed the href prop, we should deprecate <VerticalNavItem />. There is still some functionality in <VerticalNavItem /> that will need to be included elsewhere:

<Card /> doesn't have a placeholder state like <VerticalNavItem />, so it isn't as simple as simply replacing all of the instances of the latter with the former in this component. It also doesn't support an external link icon

The logic for using the external link Gridicon should live in <Card />, and we may need a new component for the placeholder state that <VerticalNavItem /> currently provides, because <Card /> probably shouldn't be opinionated about the structure of its child content.

I think any placeholder state should be created by the developer for their specific uses. If we have a generic placeholder it sort of defeats the purpose and becomes just a glorified spinner :)

VerticalNavItem has a specific style (a single line with a Gridicon on the right) that is used in domains and purchases, so creating a shared component somewhere to replicate the placeholder state (below) for this style probably makes sense if we're going to continue using it. We only use it in one component in each domains/purchases right now, so maybe we should just duplicate the CSS, or create a separate component for each section?

[gziolo]

We also use it in Domain Management section.