Add announcements (#12662)

* Add announcements

Fix #11006

* Add reactions to announcements

* Add admin UI for announcements

* Add unit tests

* Fix issues

- Add `with_dismissed` param to announcements API
- Fix end date not being formatted when time range is given
- Fix announcement delete causing reactions to send streaming updates
- Fix announcements container growing too wide and mascot too small
- Fix `all_day` being settable when no time range is given
- Change text "Update" to "Announcement"

* Fix scheduler unpublishing announcements before they are due

* Fix filter params not being passed to announcements filter

app/serializers/rest/reaction_serializer.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class REST::ReactionSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :name, :count
+
+  attribute :me, if: :current_user?
+  attribute :url, if: :custom_emoji?
+  attribute :static_url, if: :custom_emoji?
+
+  def count
+    object.respond_to?(:count) ? object.count : 0
+  end
+
+  def current_user?
+    !current_user.nil?
+  end
+
+  def custom_emoji?
+    object.custom_emoji.present?
+  end
+
+  def url
+    full_asset_url(object.custom_emoji.image.url)
+  end
+
+  def static_url
+    full_asset_url(object.custom_emoji.image.url(:static))
+  end
+end