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/system/admin_area/invoices_test.rb

@@ -0,0 +1,20 @@
+require 'test_helper'
+
+class AdminAreaInvoicesTest < ApplicationSystemTestCase
+  setup do
+    sign_in users(:admin)
+    @invoice = invoices(:one)
+  end
+
+  def test_cancels_an_invoice
+    @invoice.account_activity = nil
+    assert @invoice.cancellable?
+
+    visit admin_invoice_url(@invoice)
+    click_on 'Cancel'
+    @invoice.reload
+
+    assert @invoice.cancelled?
+    assert_text 'Invoice has been cancelled'
+  end
+end