Fix /api/v1/admin/trends/tags using wrong serializer (#18943)

* Fix /api/v1/admin/trends/tags using wrong serializer

Fix regression from #18641

* Only use `REST::Admin::TagSerializer` when the user can `manage_taxonomies`

* Fix admin trending hashtag component to not link if `id` is unknown

app/controllers/api/v1/admin/trends/tags_controller.rb

@@ -3,6 +3,14 @@
 class Api::V1::Admin::Trends::TagsController < Api::V1::Trends::TagsController
   before_action -> { authorize_if_got_token! :'admin:read' }
 
+  def index
+    if current_user&.can?(:manage_taxonomies)
+      render json: @tags, each_serializer: REST::Admin::TagSerializer
+    else
+      super
+    end
+  end
+
   private
 
   def enabled?

app/javascript/mastodon/components/admin/Trends.js

@@ -50,7 +50,7 @@ export default class Trends extends React.PureComponent {
             <Hashtag
               key={hashtag.name}
               name={hashtag.name}
-              to={`/admin/tags/${hashtag.id}`}
+              to={hashtag.id === undefined ? undefined : `/admin/tags/${hashtag.id}`}
               people={hashtag.history[0].accounts * 1 + hashtag.history[1].accounts * 1}
               uses={hashtag.history[0].uses * 1 + hashtag.history[1].uses * 1}
               history={hashtag.history.reverse().map(day => day.uses)}