Adding Notifications docs - update sample code
Adding Notifications docs - update sample code

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

jnns/wagtail,mixxorz/wagtail,zerolab/wagtail,rsalmaso/wagtail,thenewguy/wagtail,jnns/wagtail,wagtail/wagtail,thenewguy/wagtail,zerolab/wagtail,rsalmaso/wagtail,zerolab/wagtail,torchbox/wagtail,rsalmaso/wagtail,mixxorz/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,torchbox/wagtail,jnns/wagtail,thenewguy/wagtail,jnns/wagtail,zerolab/wagtail,wagtail/wagtail,mixxorz/wagtail,mixxorz/wagtail,rsalmaso/wagtail,torchbox/wagtail,wagtail/wagtail,thenewguy/wagtail,torchbox/wagtail,wagtail/wagtail,mixxorz/wagtail,zerolab/wagtail