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/models/invoice_spec.rb

@@ -1,69 +0,0 @@
-require 'rails_helper'
-
-describe Invoice do
-  context 'with invalid attribute' do
-    before :all do
-      @invoice = Invoice.new
-    end
-
-    it 'should not be valid' do
-      @invoice.valid?
-      @invoice.errors.full_messages.should match_array([
-        "Buyer name is missing",
-        "Currency is missing",
-        "Due date is missing",
-        "Invoice items is missing",
-        "Seller iban is missing",
-        "Seller name is missing",
-      ])
-    end
-
-    it 'should not have any versions' do
-      @invoice.versions.should == []
-    end
-  end
-
-  context 'with valid attributes' do
-    before :all do
-      @invoice = create(:invoice)
-    end
-
-    it 'should be valid' do
-      @invoice.valid?
-      @invoice.errors.full_messages.should match_array([])
-    end
-
-    it 'should be valid twice' do
-      @invoice = create(:invoice)
-      @invoice.valid?
-      @invoice.errors.full_messages.should match_array([])
-    end
-
-    it 'should be valid twice' do
-      @invoice = create(:invoice)
-      @invoice.valid?
-      @invoice.errors.full_messages.should match_array([])
-    end
-
-    it 'should return correct addresses' do
-      @invoice = create(:invoice)
-      @invoice.seller_address.should == 'Paldiski mnt. 123, Tallinn'
-    end
-
-    it 'should cancel overdue invoices' do
-      create(:invoice, created_at: Time.zone.now - 35.days, due_date: Time.zone.now - 30.days)
-      Invoice.cancel_overdue_invoices
-      Invoice.where(cancelled_at: nil).count.should == 1
-    end
-
-    it 'should have one version' do
-      with_versioning do
-        @invoice.versions.should == []
-        @invoice.buyer_name = 'New name'
-        @invoice.save
-        @invoice.errors.full_messages.should match_array([])
-        @invoice.versions.size.should == 1
-      end
-    end
-  end
-end