Add an option to show the sub-projects' stories in the parent project show view

[arielj]

https://ombulabs.atlassian.net/browse/DT-259

Branch/Commit:

Main branch

Describe the feature:

Maybe we can have a checkbox that will show/hide the stories of the sub-projects in the parent project show view.

Problem:

Sometimes it's useful to have view stories from the different sub-projects at the same time, currently we have to open the different sub-projects in different tabs.

Mockups:

I'm not really sure what would be a good design for this, I think it needs some discussion too, maybe it's not worth it.

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