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

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.

**I will abide by the [code of conduct](https://github.com/fastruby/points/blob/main/CODE_OF_CONDUCT.md)**

首先定位父项目的 show 视图，以及其中如何加载其 stories 和子项目。该 issue 将复选框的设计留待讨论；完成意味着已有达成一致的设计，以及一个能够在该视图中显示或隐藏子项目 stories 的可用选项。

由索引模型根据 Issue 内容生成。