post previews: fix thumbnail width/height outside post index.

Normally thumbnails have a fixed size of 154x154, but that's not always
desirable outside of the posts index because it creates empty gaps
around thumbnails.

app/javascript/src/styles/specific/posts.scss

@@ -42,6 +42,11 @@ article.post-preview {
     vertical-align: text-top;
   }
 
+  &.post-preview-compact {
+    width: auto;
+    height: auto;
+  }
+
   .desc {
     font-size: 80%;
     margin-bottom: 0;