FIX: Improve emoji upload UI (#15603)

This commit adds a hover effect for drag and drop in
the admin emoji uploader. It also changes the "Add New
Emoji" button to open the file selector; previously it
was useless because it was disabled unless a name was
entered (which is not even a requirement for the emoji)
and also it didn't actually do anything on click even
if it wasn't disabled.

Now we have a way of adding files without having to drag
and drop them, which is nice.

Also in this PR, there was no indication before that the upload was
complete apart from the button becoming enabled again.
This commit adds the highlight class to the emoji list
and removes it once the highlight fade animation is done,
like we do for new posts.

app/assets/javascripts/admin/addon/controllers/admin-emojis.js

@@ -44,6 +44,19 @@ export default Controller.extend({
     },
   }),
 
+  _highlightEmojiList() {
+    const customEmojiListEl = document.querySelector("#custom_emoji");
+    if (
+      customEmojiListEl &&
+      !customEmojiListEl.classList.contains("highlighted")
+    ) {
+      customEmojiListEl.classList.add("highlighted");
+      customEmojiListEl.addEventListener("animationend", () => {
+        customEmojiListEl.classList.remove("highlighted");
+      });
+    }
+  },
+
   @action
   filterGroups(value) {
     this.set("filter", value);
@@ -54,6 +67,7 @@ export default Controller.extend({
     emoji.url += "?t=" + new Date().getTime();
     emoji.group = group;
     this.model.pushObject(EmberObject.create(emoji));
+    this._highlightEmojiList();
   },
 
   @action