Merge branch 'issue_15673' into 'master'

Add to label ID to response

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15673

See merge request !4023

app/controllers/dashboard/labels_controller.rb

 class Dashboard::LabelsController < Dashboard::ApplicationController
   def index
-    labels = Label.where(project_id: projects).select(:title, :color).uniq(:title)
+    labels = Label.where(project_id: projects).select(:id, :title, :color).uniq(:title)
 
     respond_to do |format|
       format.json { render json: labels }