Minor tweaks for Partner user linking

A thread on the mailing list got me looking into the linking users to
partners views:
https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/django-oscar/MVdvLihnNj0/R3eecSabQjsJ

While the PartnerLinkView could programatially create the correct
permission, other parts of the codebase rely on the permission being
there. So this is not addressed, but a few issues I spotted were
remedied:
* PartnerLinkView's methods have been reordered to rough order of
  execution
* Use redirect instead of the clunky HttpResponseRedirect
* Don't mark the permission name as translatable, as it's stored in the
  database
Optional country code for Lithuanian postcodes

Per http://en.wikipedia.org/wiki/Postal_codes_in_Lithuania,
prefixing the post code with LT is valid.

Fixes #1400