Refactor and improve invoices

- `runner 'Invoice.cancel_overdue_invoices'` in `schedule.rb` is
changed to `rake 'invoices:cancel_overdue'`.
- `invoices.payment_term` database column is removed and its value is
hardcoded in UI.
- `invoices.paid_at` is removed as unused
- `invoices.due_date` column's type is now `date`.
- `Invoice#invoice_items` renamed to `Invoice#items` and `Invoice`
interface to get a list of items is unified.
- Default date format in UI.
- Default translations are used.
- Tests improved.
- Specs converted to tests and removed along with factories.
- Database structure improved.

test/services/overdue_invoice_canceller_test.rb

@@ -0,0 +1,62 @@
+require 'test_helper'
+
+class OverdueInvoiceCancellerTest < ActiveSupport::TestCase
+  setup do
+    @invoice = invoices(:one)
+  end
+
+  def test_default_delay
+    assert_equal 30.days, OverdueInvoiceCanceller.default_delay
+  end
+
+  def test_uses_default_delay_when_not_configured
+    Setting.days_to_keep_overdue_invoices_active = nil
+    canceller = OverdueInvoiceCanceller.new
+    assert_equal OverdueInvoiceCanceller.default_delay, canceller.delay
+  end
+
+  def test_uses_configured_delay
+    Setting.days_to_keep_overdue_invoices_active = 1
+    canceller = OverdueInvoiceCanceller.new
+    assert_equal 1.day, canceller.delay
+  end
+
+  def test_cancels_overdue_invoices
+    travel_to Time.zone.parse('2010-07-05')
+    @invoice.update!(account_activity: nil, cancelled_at: nil, due_date: '2010-07-03')
+    assert @invoice.cancellable?
+
+    canceller = OverdueInvoiceCanceller.new(delay: 1.day)
+    canceller.cancel
+    @invoice.reload
+
+    assert @invoice.cancelled?
+  end
+
+  def test_yields_cancelled_invoices
+    travel_to Time.zone.parse('2010-07-05')
+    @invoice.update!(account_activity: nil, cancelled_at: nil, due_date: '2010-07-03')
+    assert @invoice.cancellable?
+
+    canceller = OverdueInvoiceCanceller.new(delay: 1.day)
+
+    iteration_count = 0
+    canceller.cancel do |invoice|
+      assert_equal @invoice, invoice
+      iteration_count += 1
+    end
+    assert_equal 1, iteration_count
+  end
+
+  def test_keeps_not_overdue_invoices_intact
+    travel_to Time.zone.parse('2010-07-05')
+    @invoice.update!(account_activity: nil, cancelled_at: nil, due_date: '2010-07-04')
+    assert @invoice.cancellable?
+
+    canceller = OverdueInvoiceCanceller.new(delay: 1.day)
+    canceller.cancel
+    @invoice.reload
+
+    assert @invoice.not_cancelled?
+  end
+end