Support lazy-loading images

[CodyReichert]

For SEO and performance purposes, we should support lazy-loading images - especially in search results, widgets, and areas that show multiple listings/results at once.

WordPress supposedly has built-in support for lazy-loading images in version 5.5: https://make.wordpress.org/core/2020/07/14/lazy-loading-images-in-5-5/

I couldn't immediately get it to work (ref #196 for using <img> tags), but it probably just needs more research. This would be a valuable feature for performance.