Fix epic fullscreen editing

What does this MR do?

Fix epic fullscreen editing

Are there points in the code the reviewer needs to double check?

Shouldn't be

Why was this MR needed?

~bug

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Backport to CE https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15437
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

Closes #4065 (closed)