How to deactivate task?

User can deactivate task in the task edit form. Deactivated task will not appear on the "Today" tab, but all task data (including all task entries and notes) are kept. User can re-activate it later and keep tracking the task.

For example, after finishing study "English" for the semester, user can deactivate the task for some months, and activate it again when there is English class again.