Migrations: Add an index on the `invisible` field on users.

priv/repo/migrations/20200817120935_add_invisible_index_to_users.exs

+defmodule Pleroma.Repo.Migrations.AddInvisibleIndexToUsers do
+  use Ecto.Migration
+
+  def change do
+    create(index(:users, [:invisible]))
+  end
+end