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

The JavaScript version works while the PHP equivalent behaves differently. I have no clue why this is.

Timesheet status kept defaulting to "Open for Editing" and the value toggled between 0 and selected value. The combobox now works as expected again.

Deleting this timesheet has to be done from the timesheet details page itself.

After initial creation timesheet_period can be changed. The second time it is edited this is no longer possible. Either:
1) Fix 2nd time editing, or
2) Make checkbox non-modifiable for 2nd time opening.

BUGS:
1) Changing from "submitted" to "open for editing" by administrator doesn't work
2) Changing from "approved" to "open for editing" by adminstrator doesn't work
3) Status combobox show always "open for editing" as status even when it is one of the other statuses
4) Inserting 2 timesheets with the exact same month and year is not considered an error, while this should not be allowed.
5) Delete icon from timesheet overview doesn't work. It will forward you to timesheet details instead. You can delete the timesheet from there.

TODO:
- Remove debug statements
- Show Customer in timesheet details
- Show Customer in timesheet overview
- Show total amount of hours worked in timesheet details
- Show total amount of days worked in timesheet details
- Show total amount of days worked in timesheet overview