Make template for checkout nav/progress indicator more readable

It was rather tricky to get a grasp of what's going on in that template,
so it's been more sanely indented. The code is still rather verbose, but
that can be left for another time.

Only code change was correcting a <span> in step 3 which was outside the
{% if %} instead of in it.
Merge branch 'master' of https://github.com/tangentlabs/django-oscar into feature/demo-multipayment-with-faceted-catalogue