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/payable_test.rb

@@ -0,0 +1,53 @@
+require 'test_helper'
+
+class InvoiceTest < ActiveSupport::TestCase
+  setup do
+    @invoice = invoices(:one)
+  end
+
+  def test_unpaid_scope_returns_unpaid_invoices
+    @invoice.account_activity = nil
+    assert Invoice.unpaid.include?(@invoice), 'Should return unpaid invoice'
+  end
+
+  def test_unpaid_scope_does_not_return_paid_invoices
+    assert @invoice.paid?
+    assert_not Invoice.unpaid.include?(@invoice), 'Should not return paid invoice'
+  end
+
+  def test_paid_when_there_is_an_account_activity
+    assert @invoice.account_activity
+
+    assert @invoice.paid?
+    assert_not @invoice.unpaid?
+  end
+
+  def test_unpaid_when_there_is_no_account_activity
+    @invoice.account_activity = nil
+
+    assert @invoice.unpaid?
+    assert_not @invoice.paid?
+  end
+
+  def test_payable_when_unpaid_and_not_cancelled
+    @invoice.account_activity = nil
+    @invoice.cancelled_at = nil
+
+    assert @invoice.payable?
+  end
+
+  def test_not_payable_when_already_paid
+    assert @invoice.paid?
+    assert_not @invoice.payable?
+  end
+
+  def test_not_payable_when_cancelled
+    @invoice.cancelled_at = '2010-07-05'
+    assert_not @invoice.payable?
+  end
+
+  def test_returns_receipt_date
+    assert_equal Time.zone.parse('2010-07-05 10:00'), @invoice.account_activity.created_at
+    assert_equal Date.parse('2010-07-05'), @invoice.receipt_date
+  end
+end