Add activity expirations table

Add a table to store activity expirations. An activity can have zero or
one expirations. The expiration has a scheduled_at field which stores
the time at which the activity should expire and be deleted.

priv/repo/migrations/20190716100804_add_expirations_table.exs

@@ -0,0 +1,10 @@
+defmodule Pleroma.Repo.Migrations.AddExpirationsTable do
+  use Ecto.Migration
+
+  def change do
+    create_if_not_exists table(:activity_expirations) do
+      add(:activity_id, references(:activities, type: :uuid, on_delete: :delete_all))
+      add(:scheduled_at, :naive_datetime, null: false)
+    end
+  end
+end