Display task execution time

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

A new shortTimedelta(totalSeconds: number): string helper in method in src/helpers/date.ts takes a number of seconds and returns a short representation: mm:ss when it lasted for less than an hour, hh:mm:ss between 1 and 24 hours, and DD day(s) hh:mm:ss above that. There should be unit tests for that helper.

On the right half of the task panel in the process status view, before the buttons, the task execution time should be displayed using shortTimedelta. A title shows the exact duration using secondsToTimedelta.

The task execution time is defined as either finished - started when both are set, or Date.now() - started when only started is set. If started is not set, no time should be displayed.

Make sure that Vue does react to changes in the tasks and updates the execution time on every refresh while a task is running. Since started and finished do not change while a task runs until it finishes, I'm not sure Vue might trigger a refresh of a computed property.