Support marking a task as ended/deleted

[Supereg]

Problem

The new task model introduced with #44 currently doesn't support "ending" or deleting a task. It allows to completely delete a task version, but this might not stop a previous task version from reoccurring and might delete already recorded outcomes.

Solution

It should be possible to create "thomstone" task version. To end a task (stop if from reoccurring) you would create a new task version that is marked as deleted. This indicates that this task should never occur again.

We might not refer to this as a thombstone, as we might use the mechanisms to "pause" tasks this way (e.g., creating a "thomstone" for 2 weeks but after that adding a new version of a task that has a schedule again).

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines