DEV: {{user-selector}} replacement (#11726)

This PR is the first step towards replacing our `{{user-selector}}` and eventually deprecating and removing it from our codebase. Some of `{{user-selector}}` problems are:

1. It's called `{{user-selector}}`, but in reality in can also select groups and emails.
2. It's an Ember component, yet it doesn't have a handlebars template and uses jQuery to render itself and modify the DOM. An example of this problem is when you want to clear the selected users programmatically, see [this](6c155dba77/app/assets/javascripts/discourse/app/components/user-selector.js (L179-L185)).
3. We now have select kit which does very similar things but a lot better.

This PR introduces `{{email-group-user-chooser}}` which is meant to replace `{{user-selector}}`. It extends select kit and has the same features that `{{user-selector}}` has. `{{user-selector}}` is still used in a few places in core, but they'll all be replaced with the new component in a separate commit. 

Once `{{user-selector}}` is not used anywhere in core, it'll be deprecated and then removed after the 2.7 release.

app/assets/javascripts/admin/addon/controllers/modals/admin-merge-users-prompt.js

@@ -1,6 +1,6 @@
 import Controller, { inject as controller } from "@ember/controller";
 import ModalFunctionality from "discourse/mixins/modal-functionality";
-import { action } from "@ember/object";
+import { action, get } from "@ember/object";
 import { alias } from "@ember/object/computed";
 import discourseComputed from "discourse-common/utils/decorators";
 
@@ -27,4 +27,9 @@ export default Controller.extend(ModalFunctionality, {
   close() {
     this.send("closeModal");
   },
+
+  @action
+  updateUsername(selected) {
+    this.set("targetUsername", get(selected, "firstObject"));
+  },
 });