Add user and hub to websub client subscriptions.

lib/pleroma/web/websub/websub_client_subscription.ex

 defmodule Pleroma.Web.Websub.WebsubClientSubscription do
   use Ecto.Schema
+  alias Pleroma.User
 
   schema "websub_client_subscriptions" do
     field :topic, :string
@@ -7,6 +8,8 @@ defmodule Pleroma.Web.Websub.WebsubClientSubscription do
     field :valid_until, :naive_datetime
     field :state, :string
     field :subscribers, {:array, :string}, default: []
+    field :hub, :string
+    belongs_to :user, User
 
     timestamps()
   end

priv/repo/migrations/20170427054757_add_user_and_hub.exs

+defmodule Pleroma.Repo.Migrations.AddUserAndHub do
+  use Ecto.Migration
+
+  def change do
+    alter table(:websub_client_subscriptions) do
+      add :hub, :string
+      add :user_id, references(:users)
+    end
+  end
+end