Show rolled up weights for issues

What does this MR do and why?

Show rolled up issue weight from child tasks when feature flag is enabled. This uses our cached weights in the DB.

References

Related to #556884 (closed) / #520886 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the use_cached_rolled_up_weights feature flag.
2. Create an issue with child tasks
3. Update the weights on the tasks
4. Refresh the issue page

Child items widget should show the sum of task weights and show a progress indicator depending on closed tasks.

Real-time update will be handled separately. Note that there is also a bug where the cached weight is not updated when linking an existing task with weight. You can trigger a recompute by changing the task weight.

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