Make avatar placeholder semantic

Currently the avatar placeholder is a <p>?</p> which isn't really semantic and also allows to be selected and copied which probably isn't desired.

A better solution would be to use an empty tag like <div></div> and then use a pseudo element like

.cactus-comment-avatar > div::before {
  content: "?"
}

Centering is easy using Flexbox on the <div> (implemented by !8 (merged) on <p> already)

.cactus-comment-avatar > div {
  display: flex;
  justify-content: center;
  align-items: center;
  background: black;
  color: white;
}