FIX: Use array to keep best link for each onebox (#13717)

Use a Map to hold the best link element for each Onebox HTML element.

Using an Object did not work as intended because Object can use only
Strings or Symbols as keys. Using HTML elements (representing oneboxes)
as keys most probably converted them to some generic string and sometimes
different Oneboxes were associated same key. It seems to be browser and
content dependent, without any clear indication of what is happening
internally.

This bug caused link counts to show only for the last Onebox because
the best link from the last Onebox was considered for all the other
Oneboxes.

app/assets/javascripts/discourse/app/widgets/post-cooked.js

@@ -98,13 +98,13 @@ export default class PostCooked {
     // find the best <a> element in each onebox and display link counts only
     // for that one (the best element is the most significant one to the
     // viewer)
-    const bestElements = [];
+    const bestElements = new Map();
     $html[0].querySelectorAll("aside.onebox").forEach((onebox) => {
       // look in headings first
       for (let i = 1; i <= 6; ++i) {
         const hLinks = onebox.querySelectorAll(`h${i} a[href]`);
         if (hLinks.length > 0) {
-          bestElements[onebox] = hLinks[0];
+          bestElements.set(onebox, hLinks[0]);
           return;
         }
       }
@@ -112,7 +112,7 @@ export default class PostCooked {
       // use the header otherwise
       const hLinks = onebox.querySelectorAll("header a[href]");
       if (hLinks.length > 0) {
-        bestElements[onebox] = hLinks[0];
+        bestElements.set(onebox, hLinks[0]);
       }
     });
 
@@ -142,8 +142,8 @@ export default class PostCooked {
           const $onebox = $link.closest(".onebox");
           if (
             $onebox.length === 0 ||
-            !bestElements[$onebox[0]] ||
-            bestElements[$onebox[0]] === $link[0]
+            !bestElements.has($onebox[0]) ||
+            bestElements.get($onebox[0]) === $link[0]
           ) {
             const title = I18n.t("topic_map.clicks", { count: lc.clicks });
             $link.append(