Add loading="lazy" to <img> tag

[just1602]

We can use it in the home page, but in the article and other place like that, we could use loading="lazy" on image tag to make page load even faster.

[veganstraightedge]

Great idea!

We'll need to do that in a combination of two places:

• on any image_tag methods in this repo
• on img generation in our media_markdown gem

[just1602]

I'd do it in media markdown first, but I'd wait for the new design for doing it in the design, tho. :)