Roll call: add supervisor notification date for REST applications

In some cases external supervisors should be notified when an absence was set for a user during a roll call. Currently OpenOLAT does not know who this external supervisor is, but some external systems could know it and manage this process via REST API. 

The problem for an external system is that it must have a place to store the information wether this notification has already been sent or not. Our solution is to add a date field to the roll call of a user and make this available via REST API. 

ToDo:

• Add date field to user roll call "absenceSupervisorNotificationDate"
• Make sure the field can be read/written via REST API
• Implement a REST call to fetch all closed roll calls where a user has an absence and the new notification date is not yet set

This will an external tool allow to get a list of all new roll call with a marked absence, send a mail to whoever is the supervisor and makr the roll call in a way that the mail is not sent a second time.