Add an index on object likes

In !1538 favorites timeline was switched to use the joined object, but
no idex on likes in the joined object was added.

priv/repo/migrations/20190823000549_add_likes_index_to_objects.exs

@@ -0,0 +1,7 @@
+defmodule Pleroma.Repo.Migrations.AddLikesIndexToObjects do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists index(:objects, ["(data->'likes')"], using: :gin, name: :objects_likes)
+  end
+end