Update deprecated email configuration

Starting with version 2.6.0, configuring email notifications using EMAIL_* keys has been deprecated. Instead of providing multiple values using multiple keys, you can now provide a single URL for NOTIFICATION_URLS.

Before:

EMAIL_NOTIFICATION_RECIPIENT="you@example.com"
EMAIL_NOTIFICATION_SENDER="no-reply@example.com"
EMAIL_SMTP_HOST="posteo.de"
EMAIL_SMTP_PASSWORD="secret"
EMAIL_SMTP_USERNAME="me"
EMAIL_SMTP_PORT="587"

After:

NOTIFICATION_URLS=smtp://me:secret@posteo.de:587/?fromAddress=no-reply@example.com&toAddresses=you@example.com