Focus voucher form field when showing form

One has to click "I have a voucher" first; it makes good sense to then
give the voucher code form field focus.

Suggested by Michael in
https://groups.google.com/d/msgid/django-oscar/9ec19ae4-e9a5-47e3-96b6-80e6101ead5e%40googlegroups.com?utm_medium=email&utm_source=footer