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

db/migrate/20191218153258_create_announcements.rb

@@ -0,0 +1,16 @@
+class CreateAnnouncements < ActiveRecord::Migration[5.2]
+  def change
+    create_table :announcements do |t|
+      t.text :text, null: false, default: ''
+
+      t.boolean :published, null: false, default: false
+      t.boolean :all_day, null: false, default: false
+
+      t.datetime :scheduled_at
+      t.datetime :starts_at
+      t.datetime :ends_at
+
+      t.timestamps
+    end
+  end
+end