[FR] Subtasks (with card referencing another Card)

### Description

Hi!

I've just found the project and I'm loving it although there is a feature I'd like to suggest. It is To reference a card within a card. This would lead to have something similar to "subtasks" where one card captures and objective and needs several sub-items to accomplish it.

I think that with some small adjustments, this could be easily integrated into your workflow:

1. Make one of the status as "finilize". This would be a status that represents the final step in the workflow. Once a card has a finalize status, it can be considered as "completed" (useful for later). The user could hit a checkbox to mark a status as finalize-able and when a card is moved to that state, a hidden boolean property could be applied to it as "completed".
2. Card Referencing. This means that as a property of a card, you can input an array of card ids that this parent card would need to keep track of its "completed state". This is as simple as a loop checking if on every selected card the property is true.

This could be done through the Checklist Card Property.

The user flow would be the following:

1. Create a card indicating the parent task (do groceries)
2. Create cards with the subtasks (Buy Milk, Buy Cereal)
3. Add the subtasks cards ID to the Parent card within the List Property
4. Manage each card individually on the flow (to-do, in progress, done)
5. Parent Card would be automatically updated with a progress bar

Example in pictures



Card 2 was automatically checked as it was put on a "finalize" state (in this case, "Done").

### Impact

Every user would be benefited as this would allow the board to create subtasks that are updated automatically with little workload cost.

### Additional Context

Another approach would be that the subtasks could have the parent card ID and inform to check the corresponding box on the list state once they enter the "finalize" state.

Moreover, the state of the parent card could be automatically updated to be the minimum state (if 1 card is in done, another in progress and the other one in "to-do", the minimum would be "to-do" so the parent card is in "to-do")