activitypub: support filtering activities by whether or not they are a reply (closes #109)

priv/repo/migrations/20180617221540_create_activities_in_reply_to_index.exs

@@ -0,0 +1,8 @@
+defmodule Pleroma.Repo.Migrations.CreateActivitiesInReplyToIndex do
+  use Ecto.Migration
+  @disable_ddl_transaction true
+
+  def change do
+    create index(:activities, ["(data->'object'->>'inReplyTo')"], concurrently: true, name: :activities_in_reply_to)
+  end
+end