Add basic backend to fetch labels

- Add route and controller action for labels
- Update serializer to include id which is needed by the
frontend

ee/app/controllers/projects/integrations/jira/issues_controller.rb

@@ -45,6 +45,10 @@ module Projects
           end
         end
 
+        def labels
+          render json: issue_json[:labels]
+        end
+
         private
 
         def visitor_id

ee/app/serializers/integrations/jira_serializers/issue_entity.rb

@@ -32,6 +32,7 @@ module Integrations
       expose :labels do |jira_issue|
         jira_issue.labels.map do |name|
           {
+            id: name,
             title: name,
             name: name,
             color: '#0052CC',

ee/config/routes/project.rb

@@ -116,7 +116,11 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
 
         namespace :integrations do
           namespace :jira do
-            resources :issues, only: [:index, :show]
+            resources :issues, only: [:index, :show] do
+              member do
+                get :labels
+              end
+            end
           end
         end