Revert "feat: Switch header image to webp"

This reverts commit 6c78d5c693.

Not implemented properly - revert for now, pending MIME fix.

app/scss/ila.scss

@@ -20,7 +20,7 @@ $danger: #d9534f;
 
 
 header.microblog {
-	background: top / cover no-repeat url('/static/header-3-2.webp');
+	background: top / cover no-repeat url('/static/header-3-2.jpeg');
 	margin: 0 0 15px;
 	padding: 0 15px 3em;
 	font-size: 1.15em;