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/models/invoice/cancellable_test.rb

@@ -0,0 +1,69 @@
+require 'test_helper'
+
+class CancellableInvoiceTest < ActiveSupport::TestCase
+  setup do
+    @invoice = invoices(:one)
+  end
+
+  def test_non_cancelled_scope_returns_non_cancelled_invoices
+    @invoice.update!(cancelled_at: nil)
+    assert Invoice.non_cancelled.include?(@invoice), 'Should return cancelled invoice'
+  end
+
+  def test_non_cancelled_scope_does_not_return_cancelled_invoices
+    @invoice.update!(cancelled_at: '2010-07-05')
+    assert_not Invoice.non_cancelled.include?(@invoice), 'Should not return cancelled invoice'
+  end
+
+  def test_cancellable_when_unpaid_and_not_yet_cancelled
+    @invoice.account_activity = nil
+    @invoice.cancelled_at = nil
+    assert @invoice.cancellable?
+  end
+
+  def test_not_cancellable_when_paid
+    assert @invoice.paid?
+    assert_not @invoice.cancellable?
+  end
+
+  def test_not_cancellable_when_already_cancelled
+    @invoice.cancelled_at = '2010-07-05'
+    assert_not @invoice.cancellable?
+  end
+
+  def test_cancels_an_invoice
+    travel_to Time.zone.parse('2010-07-05 08:00')
+    @invoice.account_activity = nil
+    assert @invoice.cancellable?
+    assert_nil @invoice.cancelled_at
+
+    @invoice.cancel
+    @invoice.reload
+
+    assert @invoice.cancelled?
+    assert_equal Time.zone.parse('2010-07-05 08:00'), @invoice.cancelled_at
+  end
+
+  def test_throws_an_exception_when_trying_to_cancel_already_cancelled_invoice
+    @invoice.cancelled_at = '2010-07-05'
+
+    e = assert_raise do
+      @invoice.cancel
+    end
+    assert_equal 'Invoice cannot be cancelled', e.message
+  end
+
+  def test_not_cancelled
+    @invoice.cancelled_at = nil
+
+    assert @invoice.not_cancelled?
+    assert_not @invoice.cancelled?
+  end
+
+  def test_cancelled
+    @invoice.cancelled_at = '2010-07-05'
+
+    assert @invoice.cancelled?
+    assert_not @invoice.not_cancelled?
+  end
+end