Rename `deauth_webhook()`

[madprime]

There's a field in DataRequestProject model that is deauth_webhook, but also a method in DataRequestProjectMember by the same name.

This is minor, but it's a bit confusing to re-use the name. The method could be send_deauth_request (and it could potentially also do email notification, I don't see why it shouldn't be combined).

See: https://github.com/OpenHumans/open-humans/blob/master/private_sharing/models.py#L544