Open work items in a modal when in a modal

What does this MR do and why?

We now have an ability to view child work items from within a work item. This MR opens up the work item in a modal (as opposed to the full detail view).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Currently, the hierarchy widget is only available with OKRs. Therefore, to view this, you will need to enable the okrs_mvc feature flag

   Feature.enable(:okrs_mvc)

2. Create an objective (from the issue list view).
3. Create a key result in the objective.
4. Click the new key result.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.