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

config/initializers/simple_form.rb

@@ -98,7 +98,7 @@ SimpleForm.setup do |config|
     b.use :html5
     b.use :label
     b.use :hint, wrap_with: { tag: :span, class: :hint }
-    b.use :input
+    b.use :input, wrap_with: { tag: :div, class: :label_input }
     b.use :error, wrap_with: { tag: :span, class: :error }
   end