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/payment_orders/bank_link_test.rb

@@ -10,8 +10,8 @@ class BankLinkTest < ActiveSupport::TestCase
     @invoice = invoices(:for_payments_test)
     invoice_item = invoice_items(:one)
 
-    @invoice.invoice_items << invoice_item
-    @invoice.invoice_items << invoice_item
+    @invoice.items << invoice_item
+    @invoice.items << invoice_item
 
     travel_to '2018-04-01 00:30 +0300'
     create_new_bank_link

test/models/payment_orders/every_pay_test.rb

@@ -7,8 +7,8 @@ class EveryPayTest < ActiveSupport::TestCase
     @invoice = invoices(:for_payments_test)
     invoice_item = invoice_items(:one)
 
-    @invoice.invoice_items << invoice_item
-    @invoice.invoice_items << invoice_item
+    @invoice.items << invoice_item
+    @invoice.items << invoice_item
 
     params = {
       response: