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.

app/models/concerns/invoice/cancellable.rb

@@ -0,0 +1,28 @@
+module Concerns
+  module Invoice
+    module Cancellable
+      extend ActiveSupport::Concern
+
+      included do
+        scope :non_cancelled, -> { where(cancelled_at: nil) }
+      end
+
+      def cancellable?
+        unpaid? && not_cancelled?
+      end
+
+      def cancel
+        raise 'Invoice cannot be cancelled' unless cancellable?
+        update!(cancelled_at: Time.zone.now)
+      end
+
+      def cancelled?
+        cancelled_at
+      end
+
+      def not_cancelled?
+        !cancelled?
+      end
+    end
+  end
+end