Timesheet codes and tasks adjustments

[aharwood2]

I propose the following adjustments:

1. Timesheet tasks should not be "deletable" if they have been booked to already as this would alter historical timesheets -- a runtime check when the edit timesheet code page loads should be possible.
2. The "active" property should apply to tasks as well as activities so tasks can be switched off instead of deleted -- we can have a column with a switch as we do elsewhere to control this.