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/fixtures/account_activities.yml

@@ -1,7 +1,5 @@
-DEFAULTS: &DEFAULTS
-  account_id: 1
-
 one:
-  <<: *DEFAULTS
-  invoice: paid
+  account: cash
+  invoice: one
   bank_transaction: one
+  created_at: <%= Time.zone.parse('2010-07-05 10:00') %>

test/fixtures/accounts.yml

@@ -4,6 +4,12 @@ cash:
   currency: EUR
   registrar: bestnames
 
+two:
+  account_type: cash
+  balance: 100
+  currency: EUR
+  registrar: goodnames
+
 not_in_use_cash:
   account_type: cash
   balance: 0

test/fixtures/invoice_items.yml

@@ -1,13 +1,6 @@
 one:
   description: Acme services
   price: 5
-  amount: 1
+  quantity: 1
   unit: pc
-  invoice: valid
-
-two:
-  description: Acme services
-  price: 5
-  amount: 2
-  unit: pc
-  invoice: valid
+  invoice: one

test/fixtures/invoices.yml

@@ -1,5 +1,5 @@
-DEFAULTS: &DEFAULTS
-  created_at: <%= Date.parse '2010-07-05' %>
+one:
+  issue_date: <%= Date.parse '2010-07-05' %>
   due_date: <%= Date.parse '2010-07-06' %>
   currency: EUR
   seller_name: John Doe
@@ -10,30 +10,15 @@ DEFAULTS: &DEFAULTS
   total: 16.50
   reference_no: 13
 
-valid:
-  <<: *DEFAULTS
-
-exported:
-  <<: *DEFAULTS
-  in_directo: true
-
-cancelled:
-  <<: *DEFAULTS
-  cancelled_at: <%= Date.parse '2010-07-05' %>
-
-paid:
-  <<: *DEFAULTS
-  total: 1
-
-outstanding:
-  <<: *DEFAULTS
-  due_date: <%= Date.parse '2010-07-04' %>
-
-overdue:
-  <<: *DEFAULTS
-  due_date: <%= Date.parse '2010-07-03' %>
-
 for_payments_test:
-  <<: *DEFAULTS
-  total: 12.00
   number: 1
+  issue_date: <%= Date.parse '2010-07-05' %>
+  due_date: <%= Date.parse '2010-07-06' %>
+  currency: EUR
+  seller_name: John Doe
+  seller_iban: 1234
+  buyer: bestnames
+  buyer_name: Jane Doe
+  vat_rate: 0.1
+  reference_no: 13
+  total: 12.00