Fix subgroup url in search drop down

What does this MR do?

Inside the context of a subgroup, it fixes the url for issues and merge requests in the search drop down menu.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/52525

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.