Add feature flag to skip sidebar counts

What does this MR do and why?

We are working on moving these counts to a separate request. When this feature flag is enabled, the backend will not compute the counts and in another MR we will add logic for the frontend to fetch the counts from GraphQL

References

Please include cross links to any resources that are relevant to this MR This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #498901 (closed)
• !171238 (merged)
• !171309 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable async_sidebar_counts feature flag
2. You should no longer see issue, MR, and epic counts in the project and group sidebar

spec/lib/sidebars/groups/menus/issues_menu_spec.rb

@@ -56,6 +56,8 @@
     let(:count_service) { ::Groups::OpenIssuesCountService }
 
     before do
+      stub_feature_flags(async_sidebar_counts: false)
+
       allow_next_instance_of(count_service) do |service|
         allow(service).to receive(:count).and_raise(ActiveRecord::QueryCanceled)
       end