FIX: Sync user's reviewables count when loading reviewables list (#20128)

1. What is the problem here?

When a user's reviewables count changes, the changes are published via
MessageBus in a background Sidekiq job which means there is a delay before the
client receives the MessageBus message with the updated count. During
the time the reviewables count for a user has been updated and the time
when the client receives the MessageBus message with the updated count,
a user may view the reviewables list in the user menu. When that happens, the number of
reviewables in the list may be out of sync with the count shown.

2. What is the fix?

Going forward, the response for the `ReviewablesController#user_menu_list` action will include the user's reviewables count as
the `reviewables_count` attribute. This is then used by the client side
to update the user's reviewables count to ensure that the reviewables
list and count are kept in sync.

app/controllers/reviewables_controller.rb

@@ -81,6 +81,7 @@ class ReviewablesController < ApplicationController
           Reviewable.user_menu_list_for(current_user),
           current_user,
         ).as_json,
+      reviewable_count: current_user.reviewable_count,
     }
     render_json_dump(json, rest_serializer: true)
   end