#1 Privat flag

[Strueby]

A privat flag ("Privat Task") should be added. If it is set in a task only the user who created the task and the user that the task is assigned to can see it (in the hit list and details).

When such a task is transfered to a different user, then a warning message says that this will transfer the privacy also (user who created the task and user whom the task is assigned to).

Jumping directly to that task is only possible when one is logged in and is allowed to see the task.

The importing and updating of the database should work correctly:
* privacy not set
* user and creator correctly set.