Promote project labels to group labels

Worked live with @DouweM / @smcgivern for the initial spike.

What does this MR do?

Gives the ability for the Group Admin to promote Project Labels to Group Labels.

Are there points in the code the reviewer needs to double check?

• Icon for the promote action is needed in the Project's Labels list
• Labels from all the issues before promotion should be updated to point to the new one
• All the labels from Projects inside the Group with the same name should be merged into the new Group Label
• Label promotion requires ability to edit all the projects inside the group, Group Admin permissions is required
• Are the label priorities in each project are the same as before merge
• optional: How to resolve description during project merges. My solution for now will be - use the one which had the promote button pressed.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Screenshot

What are the relevant issue numbers?

Closes #24021 (closed)

app/services/labels/promote_service.rb

@@ -25,14 +25,14 @@ def execute(label)
 
     def prepare_for_merge_labels(label_title)
       labels_for_merge = LabelsFinder.
-        new(@current_user, { title: label_title, group_id: @project.group.id}).
+        new(@current_user, { title: label_title, group_id: @project.group.id }).
         execute(skip_authorization: true)
 
       cached_label_ids = labels_for_merge.map(&:id)
 
       # We need to nil names so they won't collide with the newly created
       # Group Label with the same title
-      labels_for_merge.update_all(title: nil)
+      Label.where(id: cached_label_ids).update_all(title: '')
 
       cached_label_ids
     end