Add streaming API updates for announcements being modified or deleted (#12963)

Change `all_day` to be a visual client-side cue only

Publish immediately if `scheduled_at` is in the past

Add `published_at` and `updated_at` to announcements JSON

app/serializers/rest/announcement_serializer.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
 class REST::AnnouncementSerializer < ActiveModel::Serializer
-  attributes :id, :content, :starts_at, :ends_at, :all_day
+  attributes :id, :content, :starts_at, :ends_at, :all_day,
+             :published_at, :updated_at
 
   has_many :mentions
   has_many :tags, serializer: REST::StatusSerializer::TagSerializer