adding indexes to oauth_tokens table

priv/repo/migrations/20190403131720_add_oauth_token_indexes.exs

+defmodule Pleroma.Repo.Migrations.AddOauthTokenIndexes do
+  use Ecto.Migration
+
+  def change do
+    create(unique_index(:oauth_tokens, [:token]))
+    create(index(:oauth_tokens, [:app_id]))
+    create(index(:oauth_tokens, [:user_id]))
+  end
+end