I had to manually correct the ru_RU django.po file as there were some
mismatching plurals. Not entirely sure what I did was the correct
resolution but it seemed sensible to my non-Russian eyes.

Mention bootstrap 3 dashboard and the customisation of the offer models

We got support for varying the tax rate per product in #1727. This
commit does some follow-up work.

Closes #1727.

This commit adds a bit of documention around the Elasticsearch support
introduced in #1572. It also shuffles around the handler names to be
more consistent.

This was added with #1550.

Building upon the previous commit, we now use ordered dicts for our
search facets.

Specifying `excludes` is deprecated in Django 1.7 and not supported in
Django 1.8.

This order offers the simplest API for common needs - as evidenced by the
fact that the apply_offers crook could be removed from two test files,
saving on imports as well.

That approach is around from sometime around Oscar 0.6. With the
category refactor, it has become expensive to support. I doubt it has
any SEO impact, as e.g. Amazon has product IDs etc. all over their URLs.
@mbertheau seconded this opinion.

I also updated the handling migration section of the general upgrading
doc to use native migrations lingo instead of South's.

Previously, all views handled in Oscar's Application class were
decorated with the permissions_required decorator. That decorator would
not do anything if used with an empty set of permissions, but it
nonetheless seems cleaner to not decorate the views in the first place.

Raised in:
https://github.com/django-oscar/django-oscar-api/pull/2/files#r23832083

Closes #1482

Part of #1569