Fix breadcrumbs for epic work items

What does this MR do and why?

Fix breadcrumbs for epic work items

Since the last argument for page_title is used for the breadcrumb_title by default, we need to override the breadcrumb_title to make sure it's using the work item reference.

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Before	After

How to set up and validate locally

Feature.enable(:namespace_level_work_items)
Feature.enable(:sync_work_item_to_epic)

1. Go to the group's epics page
2. Create an epic
3. Go to the epic's detail page.
4. It will link you to the legacy epic detail page /epics/. Change the path to /work_items/.
5. Make sure the breadcrumb is correct and shows the IID instead of "Epics"