Milestone removed on edit if Milestone was closed.

Zendesk: https://gitlab.zendesk.com/agent/tickets/18987

Problem

1. Open new milestones, let's say A and B.
2. Open new issue 444, assign to milestone A.
3. Post comments, do some work in the issue.
4. Close issue 444.
5. Close milestone A.
6. Couple days later, remember that you need to add section to the description of the issue 444 - edit issue.
7. Milestone is being removed as soon as you hit edit.

Which version affected

All known versions of GitLab are affected.

Demonstration

You can take look on sashk/test#8 (closed).

mentioned in issue #4135 (closed)

Same. When the title (or description) of closed issue is renamed, the milestone is removed.

Title changed from **Milestone removed on edit if Milestone was closed. ** to Milestone removed on edit if Milestone was closed.

Added ~113220 label

@felipe_artur or @jameslopez Can you take a look, please?

Reassigned to @felipe_artur

mentioned in merge request !3571 (merged)

Status changed to closed by commit 0913204f

mentioned in commit 0913204f

mentioned in merge request !14785