Fixes #376

* 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.