Issue on milestone page should be assigned to me when I dropped it to ongoing list

This issue was introduced with Multiple Issue Assignee

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@ClemMakesApps I prepared a regression spec https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11678. The issue should be fixed solely on FE side. You can use my MR if you want to.

How to reproduce:

  1. Create a milestone with few unassigned issues

  2. Go to the milestone page

  3. Drag the issue from the first list to the seconds like shown on the image

  4. Look at the browser console, you will see that AJAX request got 500 error.

Edited by Valery Sizov