Track task execution time

https://redmine.teklia.com/issues/8785

To limit task execution time, we need to know how long a task has been running for.

Task.started and Task.finished are two new nullable DateTimeField. Both fields are visible, but read-only, in the Django admin.

Two database constraints should require that started >= created when started is not null, and finished >= started when finished is not null, just like on Process.

RetrieveTask should include both fields in its API response. RetrieveProcess should also include both fields in the tasks of the process.

UpdateTask and PartialUpdateTask should, when a task's state is updated to Running, set Task.started to the current time. When a task's state is updated to any final state, Task.finished is set to the current time, unless Task.started is None since the constraints will block that.

The run_docker_task RQ task should set Task.started as soon as the Docker container is started. Both run_docker_task and run_task_rq should set Task.finished when reaching any final state, if Task.started is not None.