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/lib/inline_renderer.rb

@@ -15,6 +15,10 @@ class InlineRenderer
       serializer = REST::NotificationSerializer
     when :conversation
       serializer = REST::ConversationSerializer
+    when :announcement
+      serializer = REST::AnnouncementSerializer
+    when :reaction
+      serializer = REST::ReactionSerializer
     else
       return
     end