closes ENG-711
When an Admin is authenticated in Comments-UI we only add moderation options to the displayed comments so we don't need to pre-emptively load the `admin-auth` iframe and make the `/ghost/api/admin/users/me/` request until some comments are actually visible.
- used `state.comments.length` property to defer rendering of the admin auth frame until comments have been fetched (after box is scrolled into view) and the count is > 0
closes ENG-678
The comments block is typically shown at the bottom of a post so it doesn't make sense to eagerly fetch comments from the API when we don't know if the comments block will even be viewed. By lazy-loading the data only when the comments block comes into view we can reduce both data usage for visitors and load on the site.
- uses IntersectionObserver API to delay comments app initialisation until the comments block has scrolled into view
- updated all iframe-related components to forward a `ref` so we can use the `<iframe>` element reference inside the `App` component
