rename some columns
refactor to IS_BUSY
add unavailability to db when button pressed

    change unav start date and unav duration inputs views
    fix english error in comment...
fix: use UserObjectBtnModel to attach level to buttons
refactor my action performed methods
add possibility to remove a session

    add a button 'remove session' in panel admin
    add methods to presenter, controller and model to do the job
    add session retrieval to ScheduleTableComponent
    add single selection mode to table
fix: add btn 'remove session' only if user is lvl admin
add isLevelAdmin() to user and use it to check admin rights
fix compute teacher' services hours

    use native query to call teacher_service_hours
    it returns a double
add exception class to behave w/ sql exceptions
fix typing error
handle db exceptions when add session or remove unav