Drop index for 'local', it confuses the query planner.

priv/repo/migrations/20180327174350_drop_local_index_on_activities.exs

@@ -0,0 +1,7 @@
+defmodule Pleroma.Repo.Migrations.DropLocalIndexOnActivities do
+  use Ecto.Migration
+
+  def change do
+    drop index(:users, [:local])
+  end
+end