use where selector to reduce specificity for images

[tetov]

I wanted to make embedded mastodon posts made using stoot display properly in bothy. I'm using :where() since it has 0 specificity.

Before

After

[tetov]

I added video to this rule:

:where(.content, .post) is:(img, video):not(.microblog_avatar) {

for my site. Let me know if you wish to add this as a commit to this PR or to another PR.