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

spec/models/announcement_mute_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe AnnouncementMute, type: :model do
+end

spec/models/announcement_reaction_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe AnnouncementReaction, type: :model do
+end

spec/models/announcement_spec.rb

@@ -0,0 +1,4 @@
+require 'rails_helper'
+
+RSpec.describe Announcement, type: :model do
+end