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.

db/migrate/20181017092829_rename_invoice_items_amount_to_quantity.rb

@@ -0,0 +1,5 @@
+class RenameInvoiceItemsAmountToQuantity < ActiveRecord::Migration
+  def change
+    rename_column :invoice_items, :amount, :quantity
+  end
+end