Make workitems subepics behaviour same as legacy epics

What does this MR do and why?

In the case of a downgrade from ultimate, subepics should

• On the epic detail page, the add menu doesn't provide the option to add subepics.
• On the hierarchy widget within the epic detail page, the user will not see child epics (even if they exist from when they were on the ultimate tier).
• On the epic detail page, the parent widget will not display.
• From the APIs, the user cannot add or remove parent or child epics from an epic record.

Issue: #451455 (closed)

Screenshots

Before	After
video1561504977.mp4	video3561504977.mp4

How to set up and validate locally

To test this locally, you need to enable WI at group level, so you will have to enable these feature flags namespace_level_work_items (If you are still unable to access work-item page for an epic then enable/disable FF from this list - &11777 )

1. Visit Epics page and try to create a new EPIC(If all relevant FF's are enabled then you should see a modal when you click on New Epic)
2. Open the newly created epic and replace epics in the URL with work_items, SO the target URL would be something like http://127.0.0.1:3000/groups/flightjs/-/work_items/6
3. create few child issues & epics
4. Either downgrade your license from ultimate or go to https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/gitlab_subscriptions/features.rb#L263 and comment subepics
5. Upon refreshing epic page, you should not see epics in the hierarchy widget
6. Toggling step 4, should show the subepics again.
7. While you are testing the above steps, you should also notice that parent and hierarchy widget will be hidden when subepics is unavailable