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