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
    deriving Eq Ord Show
TutorialScheduleOpt
    tutorial  TutorialId
    user      UserId
    opt       Bool
    UniqueTutorialScheduleOpt tutorial user