Show blurhash only after 150ms

Blurhash on every single room entrance was extremely annoying, to the
point where it wasn't really helpful, but distracting, even after
the animations was added.
This commit makes it so the blurhash is only shown after 150ms, and the
image will be animated as a quick fade-in if there's no blurhash.

Fixes https://github.com/vector-im/element-web/issues/18617
Fixes https://github.com/vector-im/element-web/issues/17879

res/css/views/messages/_MImageBody.scss

@@ -36,6 +36,10 @@ $timelineImageBorderRadius: 4px;
         animation: mx--anim-pulse 1.75s infinite cubic-bezier(.4, 0, .6, 1);
         border-radius: $timelineImageBorderRadius;
     }
+
+    .mx_no-image-placeholder {
+        background-color: $primary-bg-color;
+    }
 }
 
 .mx_MImageBody_thumbnail_container {