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/services/overdue_invoice_canceller.rb

@@ -0,0 +1,33 @@
+class OverdueInvoiceCanceller
+  attr_reader :invoices
+  attr_reader :delay
+
+  def initialize(invoices: Invoice.overdue, delay: self.class.delay)
+    @invoices = invoices
+    @delay = delay
+  end
+
+  def self.default_delay
+    30.days
+  end
+
+  def self.delay
+    Setting.days_to_keep_overdue_invoices_active&.days || default_delay
+  end
+
+  def cancel
+    invoices.each do |invoice|
+      next unless cancellable?(invoice)
+
+      invoice.cancel
+      yield invoice if block_given?
+    end
+  end
+
+  private
+
+  def cancellable?(invoice)
+    due_date_with_delay = invoice.due_date + delay
+    due_date_with_delay.past?
+  end
+end