PERF: Refactor lightbox decorator to use querySelectorAll (#10158)

Previously we were using `$elem.find(...).not($elem.find(...))`. This took approximately 2ms on my machine with a test post.

This commit switches to using a native querySelectorAll method, which takes less than 0.5ms on the same test post.

app/assets/javascripts/discourse/app/components/image-uploader.js

@@ -86,7 +86,7 @@ export default Component.extend(UploadMixin, {
   },
 
   _applyLightbox() {
-    if (this.imageUrl) next(() => lightbox($(this.element)));
+    if (this.imageUrl) next(() => lightbox(this.element));
   },
 
   actions: {

app/assets/javascripts/discourse/app/initializers/post-decorations.js

@@ -12,7 +12,7 @@ export default {
       api.decorateCooked(highlightSyntax, {
         id: "discourse-syntax-highlighting"
       });
-      api.decorateCooked(lightbox, { id: "discourse-lightbox" });
+      api.decorateCookedElement(lightbox, { id: "discourse-lightbox" });
       if (siteSettings.support_mixed_text_direction) {
         api.decorateCooked(setTextDirections, {
           id: "discourse-text-direction"

app/assets/javascripts/discourse/app/lib/lightbox.js

@@ -6,18 +6,16 @@ import { isAppWebview, postRNWebviewMessage } from "discourse/lib/utilities";
 import { spinnerHTML } from "discourse/helpers/loading-spinner";
 import User from "discourse/models/user";
 
-export default function($elem) {
-  if (!$elem) {
+export default function(elem) {
+  if (!elem) {
     return;
   }
 
   loadScript("/javascripts/jquery.magnific-popup.min.js").then(function() {
-    const spoilers = $elem.find(".spoiler a.lightbox, .spoiled a.lightbox");
-
-    $elem
-      .find("a.lightbox")
-      .not(spoilers)
-      .magnificPopup({
+    const lightboxes = elem.querySelectorAll(
+      "*:not(.spoiler):not(.spoiled) a.lightbox"
+    );
+    $(lightboxes).magnificPopup({
       type: "image",
       closeOnContentClick: false,
       removalDelay: 300,