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.

spec/factories/invoice.rb

@@ -1,18 +0,0 @@
-FactoryBot.define do
-  factory :invoice do
-    buyer_name 'Registrar 1'
-    currency { 'EUR' }
-    due_date { Time.zone.now.to_date + 1.day }
-    seller_iban { '123' }
-    seller_name { 'EIS' }
-    seller_city { 'Tallinn' }
-    seller_street { 'Paldiski mnt. 123' }
-    vat_rate 0.2
-    buyer { FactoryBot.create(:registrar) }
-    sequence(:reference_no) { |n| "1234#{n}" }
-
-    after :build do |invoice|
-      invoice.invoice_items << FactoryBot.create_pair(:invoice_item)
-    end
-  end
-end

spec/factories/invoice_item.rb

@@ -1,8 +0,0 @@
-FactoryBot.define do
-  factory :invoice_item do
-    description { 'add money' }
-    unit 1
-    amount 1
-    price 150
-  end
-end