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/20200114113335_create_announcement_reactions.rb

@@ -0,0 +1,15 @@
+class CreateAnnouncementReactions < ActiveRecord::Migration[5.2]
+  def change
+    create_table :announcement_reactions do |t|
+      t.belongs_to :account, foreign_key: { on_delete: :cascade, index: false }
+      t.belongs_to :announcement, foreign_key: { on_delete: :cascade }
+
+      t.string :name, null: false, default: ''
+      t.belongs_to :custom_emoji, foreign_key: { on_delete: :cascade }
+
+      t.timestamps
+    end
+
+    add_index :announcement_reactions, [:account_id, :announcement_id, :name], unique: true, name: :index_announcement_reactions_on_account_id_and_announcement_id
+  end
+end