Add missing tag index on objects

The previous activity index is useless because objects are not embedded
anymore and instead a joined object is queried.

priv/repo/migrations/20190603173419_add_tag_index_to_objects.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.AddTagIndexToObjects do
+  use Ecto.Migration
+
+  def change do
+    drop_if_exists index(:activities, ["(data #> '{\"object\",\"tag\"}')"], using: :gin, name: :activities_tags)
+    create index(:objects, ["(data->'tag')"], using: :gin, name: :objects_tags)
+  end
+end