This change introduces a templatetag which assigns context for adding a
'back to search results' link to a product detail page.  This can be
useful for certain types of user (who don't use the back button) and
want to get back to their, possibly complicated, set of search filters.

Fixes #456

The profile test can_change_password checked if the password change
form can be submitted succesfully, but did not verify that Django's
authentication system accepts the new password.

Before it was lower-casing everything first which prevented users being
able to login if their email address contained capitals.  We now only
lower case the host part as this is case-insensitive.

Fixes #505

A basic security measure to ensure a user is aware if his/her account
gets compromised.  The email contains a link to the password reset form
which allows a new password to be set.

Fixes #436

Fixes #376

A user in Oscar is identified by email address instead of the
username. This is, however, not set as a ``unique`` constraint
in the user model in ``django.contrib.auth.models.User``. Checks
if an email already exists are carried out when a user registers
but are ignored when a registered user changes their
profile. This can lead to multiple users having the same email
address which should not happen.

I provide a failing test with a mixin that can be used in both
the ``UserForm`` and ``UserAndProfileForm`` to clean the email
field when validating the form. A ``ValidationError`` is raised
when a user with this email address already exists and is not
the currently edited instance (makes sure that profile updates
with unchanged email work still).

Fixes #324

* They are now known as 'stock alerts' instead of notifications.
* They have been moved into the customer app largely (instead of their
  own one).
* The model implementation has been simplified.

This is a squashed, rebased version of the original branch.

Original commit messages:

* product notifications are visible *only* if a product is not in stock
* or does not have a stock record (a new product).
* an anonymous user can sign up for a notification of a product. They
* receive a confirmation link that they have to open to activate their
* sign up. An anonymous user does not have any means of managing their
* notifications in a list. The confirmation email contains a second
* link, however, with a *unsubscribe* link that allows for disabling of
* the notification.
* a registered user can sign up for notifications without having to
* confirm it. When a registered user signs up for a notification, the
* "Notify Me" button disappears from the product information and is
* replaced by a note stating that the user has already signed up.
* registered users can manage (activate/deactivate) their notifications
* in their account settings.
* an anonymous user that has signed up for notifications and creates an
* account will pull in their notifications and have them assigned to
* their account and are then no longer marked as anonymous
* the notifications app registers a receiver for the ``post_save``
* signal of ``StockRecord``. Whenever a stock record is updated, the
* notifications are checked for this particular product and emails are
* sent out. These notifications are then disabled (not deleted) and
* marked with the date the email was sent. This hides the message from
* the registered users account. Notifications are still accessible,
* however, for staff members in the dashboard
* In the dashboard, the ``Customers`` navigation node is extended with a
* ``Notifications`` child that allows for editing, deleting and viewing
* all notifications. It also provides filtering capabilities for them
* based on status, customer name, customer email and/or product keyword.

* Tests now split into more coherent modules
* Added IDs to forms to help WebTest
* Some refactoring along the way

With complete webtest for password resetting story.