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/controllers/api/base_controller.rb

@@ -85,7 +85,7 @@ class Api::BaseController < ApplicationController
   end
 
   def require_authenticated_user!
-    render json: { error: 'This API requires an authenticated user' }, status: 401 unless current_user
+    render json: { error: 'This method requires an authenticated user' }, status: 401 unless current_user
   end
 
   def require_user!