Timesheet: Make norm-time mandatory

[#1]

As an admin, I can make it mandatory for my users to fill at least their norm-time for the timesheet period, so I can be sure that all hours are accounted for and I'm not losing out on any billable time.

The norm-time is in our case defined by the user's schedule in relation to time-sheet period. It takes into account split weeks (month change happens during the week) as well as non-working days.