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/models/backup.rb

@@ -7,11 +7,11 @@
 #  user_id           :bigint(8)
 #  dump_file_name    :string
 #  dump_content_type :string
-#  dump_file_size    :bigint
 #  dump_updated_at   :datetime
 #  processed         :boolean          default(FALSE), not null
 #  created_at        :datetime         not null
 #  updated_at        :datetime         not null
+#  dump_file_size    :bigint(8)
 #
 
 class Backup < ApplicationRecord