For some reason the base class $this->canEdit(), canDelete(), canCreate()
etc functions do not work as I expect in these situations.

The construct <?= does not work in all PHP version, so replace this
by a more generic version <?php e

Only change year to book_year when it is about a new timesheet. If the timesheet already exists use its date.

Booking for a new year now needs administrative intervention to change year.

    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