Tutorial json
    name            TutorialName
    course          CourseId
    type            TutorialType -- "Tutorium", "Zentralübung", ...
    capacity        Int Maybe -- limit for enrolment in this tutorial
    room            Text Maybe
    time            Occurrences
    regGroup        (CI Text) Maybe -- each participant may register for one tutorial per regGroup
    registerFrom    UTCTime Maybe
    registerTo      UTCTime Maybe
    deregisterUntil UTCTime Maybe
    lastChanged     UTCTime default=now()
    tutorControlled Bool default=false
    UniqueTutorial course name
    deriving Generic
Tutor
    tutorial    TutorialId
    user        UserId
    UniqueTutor tutorial user
TutorialParticipant
    tutorial    TutorialId
    user        UserId
    UniqueTutorialParticipant tutorial user
