Notification for Task status change active <> idle #19776

This issue refers to the following productboard entry: https://coder.productboard.com/detail/MTpQbUVudGl0eTphNWY2N2I5Zi02OWExLTQxYzItYmM4OS03ZTBjODI2MzhhNGM=

💁‍♂️ Problem / Opportunity

• By definition tasks may take quite a long time to be completed. We do not want the users to stare at the screen waiting because that invalidates the whole purpose.
• Some tasks may require additional data from the users. The execution will be halted waiting for refinements, or further instructions. We do not want the users to find out only after hours that the task is in fact unfinished.

📖 Solution: User Story / Epic

• As a user using Tasks I am notified about the completion of the task or a blocking prompt that has occurred within the task (i.e. when the task becomes idle).
• As a user using Tasks I am notified about the activation of the task (i.e. when the task becomes active).
• As an admin I can define the target for such notifications, in the same way as I can do this for other notifications sent by Coder.
• As a user I can decide to unsubscribe from such notifications in my account settings panel.

🎯 Definition Of Success (DOS)

• It is possible to set up notifications for tasks changing their status between active and idle in a similar way how this is done for other notifications today. The feature is tested and documented accordingly.
• We can measure the adoption of this feature.
• We expect at least 50% of Tasks users to also use Notifications for Tasks.