Introduce text and HTML representations for product attribute values

Oscar previously did neither do anything special for e.g. image
attributes or richtext attributes, nor provide hooks for doing so.

value_as_text and value_as_html properties have been introduced which
allow customising the output. Whereas the handling of the different
types is still lacking (e.g. the image type definitely needs special
handling), it is much easier to add support for that now. Adding them
has been pushed to a later date and is tracked in #1152.

Fixes #1149.
Refactor post_save signal for users

Ticket #1127 correctly mentioned that we can't use Django's
get_user_model to register signals:
https://docs.djangoproject.com/en/1.6/topics/auth/customizing/#custom-users-and-signals

This commit rewrites the affected migrate_alerts_to_user receiver to
just happen in Oscar's AbstractUser save() method. This works upwards of
Django 1.4. For Django 1.4, a receiver must be manually registered to
the correct User model.
Re-bind image formset if other product formsets fail to validate

We currently don't hold on to images that are uploaded if any of the
other formsets fail when editing/creating products. To prevent
issue #1126, we need to re-bind the formset.

This behaviour should be improved as part of #856.
Ensure basket quantity links don't wrap

Part of #1148
[Backport] Respect reviews feature flag on anonymous order status page

Reviews weren't hidden and hence the URL reversing failed.

Fixes #1133.

(cherry picked from commit 0b5b8f365ce310ef137a88384045109a0169f0c9)
[Backport] Use unicode for AdvanceSelect widget

The widget returned Python 2's string type, but should return unicode.

Fixes #1134.

(cherry picked from commit 375a8f451538d0b426c8a8ddad8c37b1be1e8ceb)
[Backport] String fixes for wish lists

One string wasn't marked for translation. And 'Edit' was unclear,
because 'view'ing a wish list would allow one to edit it's products. So
'Edit' has been renamed to 'wish list settings', where one currently can
only change the name.

(cherry picked from commit b0b5dacab52620c3efe90a6daf376fc42646a6a5)
[Backport] Fix disappearing mobile menu

There's a bug in Bootstrap which affects the navbar when it's collapsed:
https://github.com/twbs/bootstrap/issues/11145
This commit fixes the issue.

Fixes #1118, #1109

(cherry picked from commit 2e72dd07c240e80c30eb5207da88e572aaca3e1d)
[Backport] Add confirmation view for removing products from wish lists

We can't always POST to the view, so an optional confirmation view was
added.

Fixes #1121

(cherry picked from commit 52821db3236ff735f43db461c4e72f3c41eeeec5)
Update release notes with changes in recent backports
Gracefully handle deleted products

This affected:

- Order history
- Reviews dashboard
- Wishlist detail

Fixes #1146
Related to #1121
Update .mailmap for 0.6.1 release
Pull latest Transifex .po files

It looks like 0.6 was missing some translation files due to a bug in the
Transifex client which was tricked by local timestamps and skipping some
translation files:
https://github.com/transifex/transifex-client/issues/22

We need to remember to use `tx pull -af` next time to force the download
of .po files.
Bump version to 0.6.1
Merge branch 'releases/0.6' of https://github.com/tangentlabs/django-oscar into releases/0.6