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/helpers/admin/announcements_helper.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Admin::AnnouncementsHelper
+  def time_range(announcement)
+    if announcement.all_day?
+      safe_join([l(announcement.starts_at.to_date), ' - ', l(announcement.ends_at.to_date)])
+    else
+      safe_join([l(announcement.starts_at), ' - ', l(announcement.ends_at)])
+    end
+  end
+end