Cascade site deletion to invitations

priv/repo/migrations/20210629124428_cascade_site_deletion_to_invitations.exs

+defmodule Plausible.Repo.Migrations.CascadeSiteDeletionToInvitations do
+  use Ecto.Migration
+
+  def change do
+    drop constraint("invitations", "invitations_site_id_fkey")
+
+    alter table(:invitations) do
+      modify :site_id, references(:sites, on_delete: :delete_all), null: false
+    end
+  end
+end