Make user properties used in notification email configurable

Emails sent via notifications use the modifiers firstname and lastname hardcoded. In some setups it would also be usefull to have the email, username oder other user properties.

To make this configurable the following has to be done:

• Add user properties with key org.olat.core.commons.services.notifications.NotificationHelper to file src/main/java/org/olat/user/propertyhandlers/_spring/userPropertiesContext.xml
• Add translation key user.formatted to translation package src/main/java/org/olat/core/commons/services/notifications/ui/_i18n/