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

lib/tasks/auto_annotate_models.rake

@@ -4,6 +4,7 @@ if Rails.env.development?
   task :set_annotation_options do
     Annotate.set_defaults(
       'routes'                  => 'false',
+      'models'                  => 'true',
       'position_in_routes'      => 'before',
       'position_in_class'       => 'before',
       'position_in_test'        => 'before',