[Task Detail] Implement task complete behaviour

[shorodilov]

Description

The detail view can contain one of two buttons:

• complete
• reopen

Complete button has CSS class btn-outline-success, and aria-label=complete.
Reopen button has CSS class btn-outline-warning, and aria-label=reopen.

Task header also has additional checkmark for completed tasks.

Blocked by

• [Task Detail] Add task detail view sources #2

DoD

• When user clicks complete button, the task status is swapped.