Promote project label into existing group label

• Currently, if you have a project label and that project's parent group has a group label with the same name, GitLab does not let you promote the project label into the group label.

• The change in this issue is to allow this.

• There would be no confusion to allow this. Because issues and mrs and epics that have that parent group label would not be affected.

• This solves the edge case outlined in: https://gitlab.com/gitlab-org/gitlab-ce/issues/45417#note_68772565

• Currently, if you have a group label with name label, you cannot create a project label within a project of that group, with the name label. This restriction does not need to change. It can be. But that can be discussed in another issue.