Add tag index.

This still isn't optimal, but at least it isn't super slow.

priv/repo/migrations/20170917120416_add_tag_index.exs

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