FIX: Remove tags from experimental sidebar on notification level changed (#17083)

As part of this commit, a bug where updating a tag's notification level on the server side does not update the state of the user's tag notification levels on the client side is fixed too.

app/serializers/concerns/user_tag_notifications_mixin.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+module UserTagNotificationsMixin
+  def muted_tags
+    tags_with_notification_level(:muted)
+  end
+
+  def tracked_tags
+    tags_with_notification_level(:tracking)
+  end
+
+  def watching_first_post_tags
+    tags_with_notification_level(:watching_first_post)
+  end
+
+  def watched_tags
+    tags_with_notification_level(:watching)
+  end
+
+  def regular_tags
+    tags_with_notification_level(:regular)
+  end
+
+  def tags_with_notification_level(lookup_level)
+    tag_user_notification_levels.select do |id, level|
+      level == TagUser.notification_levels[lookup_level]
+    end.keys
+  end
+
+  def tag_user_notification_levels
+    @tag_user_notification_levels ||= TagUser.notification_levels_for(user)
+  end
+end