Remove ability to create duplicate label names across project/group levels

Problem to solve

Duplicating labels has created a few issues that now need resolving:

We need to improve the user experience of managing labels.

Proposal

We don't encourage the creation of duplicate labels - and instead encourage use of the highest level group label. If we suggest this:

Then once they click "add default lists" - GitLab should use the highest group level label rather than create new project labels with the same names as the group(s).

If a user tries to create a new label and there is already a group label with that name, the system should prevent them from creating the duplicate label and insist they use the group label.
If a user tries to create a new label on the group level and there is already an existing label with the same name in a child project, the system should insist that they go promote that label to the top most parent group.

There may be some issues with permissions here - i.e. managing labels at a group level when a user doesn't have permission to do that but wants to manage labels in their own project.

What does success look like?

Cleaner label management and less risk of creating bugs where duplicate labels conflict.
Users are never able to create a label with the same name anywhere within a top level group and all of its descendant groups and projects.

Edited Jul 02, 2020 by Gabe Weaver