Use new PARTNER_NOTIFICATION_ADDRESS instead of old PARTNER_EMAIL

Ghost User
2019-08-01T13:43:05+02:00

`PARTNER_EMAIL` was replaced in a393f6e9ec591cd61991fd888729bf3cd0140d78 by `PARTNER_NOTIFICATION_ADDRESS`.

#### Previous behaviour
Steps to reproduce:
[Sentry Issue](https://sentry.io/organizations/thalia/issues/1134801598/?project=1470866)