Update sample code in the section
Extending Wagtail -> Adding new Task types -> Adding notifications
The previous example did not work because of changes in
`wagtail.admin.mail`
`EmailNotifier` doesn't exist, so we need to import
`EmailNotificationMixin` and `Notifier` instead, and
update `BaseUserApprovalTaskStateEmailNotifier`
accordingly
These were previously added at Daniele Procida's recommendation so that 'how to' pages could easily be identified in a future reorganisation that splits them out from other modes of documentation. Since 'Extending Wagtail' as a whole is expressly a 'how to' section, this is no longer required.
