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/templates/api-keys-new.hbs

@@ -25,10 +25,14 @@
 
     {{#if showUserSelector}}
       {{#admin-form-row label="admin.api.user"}}
-        {{user-selector single="true"
-          usernames=model.username
-          placeholderKey="admin.api.user_placeholder"
-          }}
+        {{email-group-user-chooser
+          value=model.username
+          onChange=(action "updateUsername")
+          options=(hash
+            maximum=1
+            filterPlaceholder="admin.api.user_placeholder"
+          )
+        }}
       {{/admin-form-row}}
     {{/if}}
 

app/assets/javascripts/admin/addon/templates/email-preview-digest.hbs

@@ -5,7 +5,13 @@
     <label for="last-seen">{{i18n "admin.email.last_seen_user"}}</label>
     {{date-picker-past value=lastSeen id="last-seen"}}
     <label>{{i18n "admin.email.user"}}:</label>
-    {{user-selector single="true" usernames=username canReceiveUpdates=true}}
+    {{email-group-user-chooser
+      value=username
+      onChange=(action "updateUsername")
+      options=(hash
+        maximum=1
+      )
+    }}
     {{d-button
       class="btn-primary digest-refresh-button"
       action=(action "refresh")

app/assets/javascripts/admin/addon/templates/modal/admin-merge-users-prompt.hbs

@@ -1,10 +1,15 @@
 <div>
   {{#d-modal-body rawTitle=(i18n "admin.user.merge.prompt.title" username=username)}}
     <p>{{html-safe (i18n "admin.user.merge.prompt.description" username=username)}}</p>
-    {{user-selector single=true
-                    placeholderKey="admin.user.merge.prompt.target_username_placeholder"
-                    usernames=targetUsername
-                    autocomplete="discourse"}}
+    {{email-group-user-chooser
+      value=targetUsername
+      autocomplete="discourse"
+      onChange=(action "updateUsername")
+      options=(hash
+        maximum=1
+        filterPlaceholder="admin.user.merge.prompt.target_username_placeholder"
+      )
+    }}
   {{/d-modal-body}}
 
   <div class="modal-footer">