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.

db/migrate/20181017092829_rename_invoice_items_amount_to_quantity.rb

@@ -0,0 +1,5 @@
+class RenameInvoiceItemsAmountToQuantity < ActiveRecord::Migration
+  def change
+    rename_column :invoice_items, :amount, :quantity
+  end
+end

db/migrate/20181017153658_add_invoice_items_invoice_id_fk.rb

@@ -0,0 +1,5 @@
+class AddInvoiceItemsInvoiceIdFk < ActiveRecord::Migration
+  def change
+    add_foreign_key :invoice_items, :invoices, name: 'invoice_items_invoice_id_fk'
+  end
+end

db/migrate/20181017153812_change_invoice_items_invoice_id_to_not_null.rb

@@ -0,0 +1,5 @@
+class ChangeInvoiceItemsInvoiceIdToNotNull < ActiveRecord::Migration
+  def change
+    change_column_null :invoice_items, :invoice_id, false
+  end
+end

db/migrate/20181017153935_change_invoice_items_quantity_to_not_null.rb

@@ -0,0 +1,5 @@
+class ChangeInvoiceItemsQuantityToNotNull < ActiveRecord::Migration
+  def change
+    change_column_null :invoice_items, :quantity, false
+  end
+end

db/migrate/20181017154038_change_invoice_items_unit_to_not_null.rb

@@ -0,0 +1,5 @@
+class ChangeInvoiceItemsUnitToNotNull < ActiveRecord::Migration
+  def change
+    change_column_null :invoice_items, :unit, false
+  end
+end

db/migrate/20181017154143_change_invoice_items_price_to_not_null.rb

@@ -0,0 +1,5 @@
+class ChangeInvoiceItemsPriceToNotNull < ActiveRecord::Migration
+  def change
+    change_column_null :invoice_items, :price, false
+  end
+end

db/migrate/20181017205123_change_invoices_due_date_to_date.rb

@@ -0,0 +1,5 @@
+class ChangeInvoicesDueDateToDate < ActiveRecord::Migration
+  def change
+    change_column :invoices, :due_date, :date, null: false
+  end
+end

db/migrate/20181022100114_add_invoices_issue_date.rb

@@ -0,0 +1,5 @@
+class AddInvoicesIssueDate < ActiveRecord::Migration
+  def change
+    add_column :invoices, :issue_date, :date
+  end
+end

db/migrate/20190311111718_remove_invoices_paid_at.rb

@@ -0,0 +1,5 @@
+class RemoveInvoicesPaidAt < ActiveRecord::Migration
+  def change
+    remove_column :invoices, :paid_at
+  end
+end

db/migrate/20190312211614_remove_invoices_payment_term.rb

@@ -0,0 +1,5 @@
+class RemoveInvoicesPaymentTerm < ActiveRecord::Migration
+  def change
+    remove_column :invoices, :payment_term
+  end
+end