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

db/structure.sql

@@ -1039,11 +1039,11 @@ ALTER SEQUENCE public.epp_sessions_id_seq OWNED BY public.epp_sessions.id;
 
 CREATE TABLE public.invoice_items (
     id integer NOT NULL,
-    invoice_id integer,
+    invoice_id integer NOT NULL,
     description character varying NOT NULL,
-    unit character varying,
-    amount integer,
-    price numeric(10,2),
+    unit character varying NOT NULL,
+    quantity integer NOT NULL,
+    price numeric(10,2) NOT NULL,
     created_at timestamp without time zone,
     updated_at timestamp without time zone,
     creator_str character varying,
@@ -1078,13 +1078,11 @@ CREATE TABLE public.invoices (
     id integer NOT NULL,
     created_at timestamp without time zone NOT NULL,
     updated_at timestamp without time zone NOT NULL,
-    due_date timestamp without time zone NOT NULL,
-    payment_term character varying,
+    due_date date NOT NULL,
     currency character varying NOT NULL,
     description character varying,
     reference_no character varying NOT NULL,
     vat_rate numeric(4,3),
-    paid_at timestamp without time zone,
     seller_id integer,
     seller_name character varying NOT NULL,
     seller_reg_no character varying,
@@ -1118,7 +1116,8 @@ CREATE TABLE public.invoices (
     cancelled_at timestamp without time zone,
     total numeric(10,2) NOT NULL,
     in_directo boolean DEFAULT false,
-    buyer_vat_no character varying
+    buyer_vat_no character varying,
+    issue_date date
 );
 
 
@@ -4234,6 +4233,14 @@ ALTER TABLE ONLY public.account_activities
     ADD CONSTRAINT fk_rails_d2cc3c2fa9 FOREIGN KEY (price_id) REFERENCES public.prices(id);
 
 
+--
+-- Name: invoice_items_invoice_id_fk; Type: FK CONSTRAINT; Schema: public; Owner: -
+--
+
+ALTER TABLE ONLY public.invoice_items
+    ADD CONSTRAINT invoice_items_invoice_id_fk FOREIGN KEY (invoice_id) REFERENCES public.invoices(id);
+
+
 --
 -- Name: messages_registrar_id_fk; Type: FK CONSTRAINT; Schema: public; Owner: -
 --
@@ -4926,6 +4933,22 @@ INSERT INTO schema_migrations (version) VALUES ('20181001090536');
 
 INSERT INTO schema_migrations (version) VALUES ('20181002090319');
 
+INSERT INTO schema_migrations (version) VALUES ('20181017092829');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017153658');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017153812');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017153935');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017154038');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017154143');
+
+INSERT INTO schema_migrations (version) VALUES ('20181017205123');
+
+INSERT INTO schema_migrations (version) VALUES ('20181022100114');
+
 INSERT INTO schema_migrations (version) VALUES ('20181108154921');
 
 INSERT INTO schema_migrations (version) VALUES ('20181129150515');
@@ -4962,3 +4985,7 @@ INSERT INTO schema_migrations (version) VALUES ('20190102115333');
 
 INSERT INTO schema_migrations (version) VALUES ('20190102144032');
 
+INSERT INTO schema_migrations (version) VALUES ('20190311111718');
+
+INSERT INTO schema_migrations (version) VALUES ('20190312211614');
+