BUGS:
    1) fixed
    2) fixed
    3) fixed
    4) Inserting 2 timesheets with the exact same month and year is not considered an error, while this should not be allowed.
    5) fixed by removing delete icon from timesheet overview page

    TODO:
    - Fix changing timesheet_period selection on second entry of timesheet
    - Show total amount of days worked in timesheet details
    - Show total amount of days worked in timesheet overview