fix: Fix Backlinks not applying the display class (#519)

* fix: Fix `Backlinks` not applying the display class

Fix #518

* fix: Apply `displayClass` to all layout components

* refactor: Use same style

* fix: Remove `undefined` class using coalescing operator

quartz/components/TableOfContents.tsx

@@ -19,7 +19,7 @@ function TableOfContents({ fileData, displayClass }: QuartzComponentProps) {
   }
 
   return (
-    <div class={`toc ${displayClass}`}>
+    <div class={`toc ${displayClass ?? ""}`}>
       <button type="button" id="toc">
         <h3>Table of Contents</h3>
         <svg