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_item_test.rb

@@ -0,0 +1,8 @@
+require 'test_helper'
+
+class InvoiceItemTest < ActiveSupport::TestCase
+  def test_calculates_sum_without_vat
+    invoice_item = InvoiceItem.new(price: 5, quantity: 2)
+    assert_equal 10, invoice_item.item_sum_without_vat
+  end
+end