Allow labels to be changed via a dropdown like assignee/milestones

• Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/11361

It would be helpful to have labels that can be added in a dropdown similar to how assignees and milestones are managed from the issue/merge request page.

The actual requirement is to be able to modify labels even after a MR is merged, but this should resolve that issue. I also believe this is something we were wanting to implement anyway but could not find an issue for it.

@DouweM Can you estimate the time required, please?

mentioned in issue #3279 (closed)

@dblessing I think this is a duplicate of #2785 (closed) and #3331 (closed)

Status changed to closed

Duplicate of #2785 (closed).

Thanks @razer6 Searching is hard haha

I knew it because there was another dup these days...

marked this issue as related to #37999 (closed)

removed the relation with #37999 (closed)

mentioned in merge request !14785