Allow instance administrators to link work items

What does this MR do and why?

Currently we only allow group/project members to link work items, but instance level administrators need to be able to do so as well.

Related to: Give instance level admins permissions to link group level issues

Screenshots or screen recordings

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Before	After
Instance administrators cannot link group level work items unless they are added as namespace members	Administrators can link group level work items

How to set up and validate locally

1. Enable the namespace_level_work_items feature flag: `Feature.enabled?(:namespace_level_work_items)`
2. As a non-admin user, create a group and navigate to its work item Plan->Epics
3. Add two epics
4. Log in as an instance level administrator
5. When you open one of the epics you will not be given the option to link it to other epics
6. When checked out to this branch, the 'Add' button should appear.