Migrations: Add index on client_id and client_secret for apps.

Greatly speeds up app lookup.

priv/repo/migrations/20200526144426_add_apps_indexes.exs

@@ -0,0 +1,7 @@
+defmodule Pleroma.Repo.Migrations.AddAppsIndexes do
+  use Ecto.Migration
+
+  def change do
+    create(index(:apps, [:client_id, :client_secret]))
+  end
+end