Fix permissions to relate work items

What does this MR do and why?

We should allow guests to link work items to each other. For epics, we only allow linking when the related_epics feature is enabled.

Screenshots or screen recordings

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Before	After

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 a guest, or instance 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.