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
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