Fix milestone page assignees when dropped in ongoing list

What does this MR do?

• Fix milestone page assignees when dropped in the "Ongoing Issues (open and assigned)" list

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

1. Get some issues added to a milestone
2. Visit the milestone page http://localhost:3000/gitlab-org/gitlab-ce/milestones/4
3. Drag an issue into the "Ongoing Issues (open and assigned)" list
4. Notice the assignee avatar shows up and on refresh maintains it's position

---

Other MRs to remove legacy singular assignee code

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12077

Why was this MR needed?

• Dropping an issue in the "Ongoing Issues (open and assigned)" would put it there but not actually add you as an assignee and on refresh, would jump back to unstarted list.

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Thanks @vsizov for the test, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11678/

Closes #32861 (closed)