useLazyLoading: start loading when near viewport
useLazyLoading: start loading when near viewport

## Issue
#6332 <Thumbnail lazy-loader is too slow || Use browser-level lazy-loading>

## Change
Switch from "threshold-based check" to "viewport distance comparison" using the `rootMargin` parameter. The root is the viewport.

This change makes it closer to the native `loading="lazy"` behavior, where it starts to load when approaching the viewport. Chrome I believe uses 3000px distance -- I think 500px is a good compromise for now. Can adjust further.

## Future
- We are currently creating N instances of IntersectionObserver.
  - https://developers.google.com/web/updates/2016/04/intersectionobserver
  - "If you need to observe multiple elements, it is both possible and advised to observe multiple elements using the same IntersectionObserver instance by calling observe() multiple times."

This would probably need a refactor to make ClaimList (or something higher) own the IntersectionObserver.

lbryio/lbry-app,lbryio/lbry-app