Little optimisation of sending alerts to users.
Do not connection when no emails to send.

Product was not resolving properly due to context change.

Fixes #342
Fixes #338

Wasn't opening the connection first (according to Django's docs).  Not
100% this works - we'll see in the sandbox.

* 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.