Fix migration.

priv/repo/migrations/20180327175831_actually_drop_local_index.exs

+defmodule Pleroma.Repo.Migrations.ActuallyDropLocalIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:users, [:local])
+    drop_if_exists index("activities", :local)
+  end
+end