Add index on inReplyTo for objects

Fixes the performance of `get_existing_votes`

priv/repo/migrations/20190603162018_add_object_in_reply_to_index.exs

@@ -0,0 +1,7 @@
+defmodule Pleroma.Repo.Migrations.AddObjectInReplyToIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:objects, ["(data->>'inReplyTo')"], name: :objects_in_reply_to_index)
+  end
+end