Fix PostgreSQL load when linking in announcements (#13250)

* Fix PostgreSQL load when linking in announcements

Fixes #13245 by caching status lookups

Since statuses are supposed to be known already and we only
need their URLs and a few other things, caching them should
be fine.

Since it's only used by announcements so far, there won't
be much statuses to cache.

* Perform status lookup when saving announcements, not when rendering them

* Change EntityCache#status to fetch URLs instead of looking into the database

* Move announcement link lookup to publishing worker

* Address issues pointed out during review

db/migrate/20200312162302_add_status_ids_to_announcements.rb

@@ -0,0 +1,6 @@
+class AddStatusIdsToAnnouncements < ActiveRecord::Migration[5.2]
+  def change
+    add_column :announcements, :status_ids, :bigint, array: true
+  end
+end
+

db/schema.rb

@@ -231,6 +231,7 @@ ActiveRecord::Schema.define(version: 2020_03_12_185443) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.datetime "published_at"
+    t.bigint "status_ids", array: true
   end
 
   create_table "backups", force: :cascade do |t|