Serve basic pages from content-store

Get an item from the content store and display it on a page. Views are very basic at this point, and controller just passes the content hash directly to the page.

More idiomatic Rails (assuming the routing is updated as described below) would be:

```ruby
def show
  if @content_item = content_store.content_item(params[:path])
    set_expiry(@content_item.expires_in)
  else
    render text: "Not found", status: :not_found
  end
end
```