add activities likes index, fixes favorites API call

priv/repo/migrations/20190127151220_add_activities_likes_index.exs

+defmodule Pleroma.Repo.Migrations.AddActivitiesLikesIndex do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def change do
+    create index(:activities, ["((data #> '{\"object\",\"likes\"}'))"], concurrently: true, name: :activities_likes, using: :gin)
+  end
+end