Add role_invoices

This new role allows users to create invoices without role_finance. Users
can then only modify their own created invoices until somebody with the
role_finance sets the paid_on value.

db/migrate/20160217194036_add_role_invoices_to_group.rb

@@ -0,0 +1,5 @@
+class AddRoleInvoicesToGroup < ActiveRecord::Migration
+  def change
+    add_column :groups, :role_invoices, :boolean, :default => false, :null => false
+  end
+end