Add Hour, FourHours to Period. Add companion object
Add postpone field to Task
Update Postpone button to use new Period companion object
Explicit TaskListView#checkedItems return type
Postpone button takes taskList as parameter

To get to the checked items
Show postponed tasks in completed tasks list

Add postpone to TaskAdapter filters
Merge branch 'feature/postponing-tasks' into dev

* feature/postponing-tasks:
  Show postponed tasks in completed tasks list
  Postpone button takes taskList as parameter
  Explicit TaskListView#checkedItems return type
  Update Postpone button to use new Period companion object
  Add postpone field to Task
  Add Hour, FourHours to Period. Add companion object