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/Footer.tsx

@@ -1,4 +1,4 @@
-import { QuartzComponentConstructor } from "./types"
+import { QuartzComponentConstructor, QuartzComponentProps } from "./types"
 import style from "./styles/footer.scss"
 import { version } from "../../package.json"
 
@@ -7,11 +7,11 @@ interface Options {
 }
 
 export default ((opts?: Options) => {
-  function Footer() {
+  function Footer({ displayClass }: QuartzComponentProps) {
     const year = new Date().getFullYear()
     const links = opts?.links ?? []
     return (
-      <footer>
+      <footer class={`${displayClass ?? ""}`}>
         <hr />
         <p>
           Created with <a href="https://quartz.jzhao.xyz/">Quartz v{version}</a>, © {year}