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.

lib/tasks/data_migrations/populate_invoice_issue_date.rake

@@ -0,0 +1,16 @@
+namespace :data_migrations do
+  task populate_invoice_issue_date: [:environment] do
+    processed_invoice_count = 0
+
+    Invoice.transaction do
+      Invoice.find_each do |invoice|
+        invoice_issue_date = invoice.created_at.to_date
+        invoice.update!(issue_date: invoice_issue_date)
+
+        processed_invoice_count += 1
+      end
+    end
+
+    puts "Invoices processed: #{processed_invoice_count}"
+  end
+end