Add a combined statistic for tasks

While it is possible to view the statistics for a single type of tasks, it is not possible to view the combined total of completed tasks.

It could be a possibility to make the id in [TaskController::fetchStats](https://github.com/schmop/cleanly-server/blob/1e26d41ff9a39125e1879aed01b637ef1b4601e2/src/Controller/TaskController.php#L118) nullable and return the total if no task was specified.