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/announcement_mute.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+# == Schema Information
+#
+# Table name: announcement_mutes
+#
+#  id              :bigint(8)        not null, primary key
+#  account_id      :bigint(8)
+#  announcement_id :bigint(8)
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#
+
+class AnnouncementMute < ApplicationRecord
+  belongs_to :account
+  belongs_to :announcement, inverse_of: :announcement_mutes
+
+  validates :account_id, uniqueness: { scope: :announcement_id }
+end