Add basic mastodon notification support.

priv/repo/migrations/20170911123607_create_notifications.exs

@@ -0,0 +1,15 @@
+defmodule Pleroma.Repo.Migrations.CreateNotifications do
+  use Ecto.Migration
+
+  def change do
+    create table(:notifications) do
+      add :user_id, references(:users, on_delete: :delete_all)
+      add :activity_id, references(:activities, on_delete: :delete_all)
+      add :seen, :boolean, default: false
+
+      timestamps()
+    end
+
+    create index(:notifications, [:user_id])
+  end
+end