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/models/bank_transaction_test.rb

@@ -2,7 +2,7 @@ require 'test_helper'
 
 class BankTransactionTest < ActiveSupport::TestCase
   def test_matches_against_invoice_reference_number
-    invoices(:valid).update!(number: '2222', total: 10, reference_no: '1111')
+    invoices(:one).update!(account_activity: nil, number: '2222', total: 10, reference_no: '1111')
     transaction = BankTransaction.new(description: 'invoice #2222', sum: 10, reference_no: '1111')
 
     assert_difference 'AccountActivity.count' do
@@ -20,7 +20,7 @@ class BankTransactionTest < ActiveSupport::TestCase
   end
 
   def test_underpayment_is_not_matched_with_invoice
-    invoices(:valid).update!(number: '2222', total: 10)
+    invoices(:one).update!(account_activity: nil, number: '2222', total: 10)
     transaction = BankTransaction.new(sum: 9)
 
     assert_no_difference 'AccountActivity.count' do
@@ -30,7 +30,7 @@ class BankTransactionTest < ActiveSupport::TestCase
   end
 
   def test_overpayment_is_not_matched_with_invoice
-    invoices(:valid).update!(number: '2222', total: 10)
+    invoices(:one).update!(account_activity: nil, number: '2222', total: 10)
     transaction = BankTransaction.new(sum: 11)
 
     assert_no_difference 'AccountActivity.count' do
@@ -40,7 +40,7 @@ class BankTransactionTest < ActiveSupport::TestCase
   end
 
   def test_cancelled_invoice_is_not_matched
-    invoices(:valid).update!(number: '2222', total: 10, cancelled_at: '2010-07-05')
+    invoices(:one).update!(account_activity: nil, number: '2222', total: 10, cancelled_at: '2010-07-05')
     transaction = BankTransaction.new(sum: 10)
 
     assert_no_difference 'AccountActivity.count' do