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/tasks/data_migrations/populate_invoice_issue_date_test.rb

@@ -0,0 +1,37 @@
+require 'test_helper'
+
+class PopulateInvoiceIssueDateTaskTest < ActiveSupport::TestCase
+  setup do
+    @invoice = invoices(:one)
+  end
+
+  def test_populates_invoice_issue_date
+    eliminate_effect_of_other_invoices
+    @invoice.update_columns(issue_date: nil, created_at: Time.zone.parse('2010-07-05'))
+    assert_nil @invoice.read_attribute(:issue_date)
+
+    capture_io { run_task }
+    @invoice.reload
+
+    assert_equal Date.parse('2010-07-05'), @invoice.issue_date
+  end
+
+  def test_outputs_results
+    eliminate_effect_of_other_invoices
+    @invoice.update_columns(issue_date: nil, created_at: Time.zone.parse('2010-07-05'))
+
+    assert_output("Invoices processed: 1\n") { run_task }
+  end
+
+  private
+
+  def eliminate_effect_of_other_invoices
+    Invoice.connection.disable_referential_integrity do
+      Invoice.delete_all("id != #{@invoice.id}")
+    end
+  end
+
+  def run_task
+    Rake::Task['data_migrations:populate_invoice_issue_date'].execute
+  end
+end