Use `MutationObserver` to automatically find new images injected into the page

[13twelve]

Could possibly replace, or at least assist (by automating) the requirement to document.addEventListener('page:updated', () => lazyloading.reset()); when injecting new content.

MutationObserver could be used to add entries to the IntersectionObserver when injected into the page and also to remove entries from the IntersectionObserver if content is removed from the page.

A17 Behaviors uses one - I think the implementation will be similar here.