FIX: sync reviewable count when opening the hamburger menu (#10368)

When a tab is open but left unattended for a while, the red, green, and blue
pills tend to go out of sync.

So whevener we open the notifications menu, we sync up the notification count
(eg. blue and green pills) with the server.

However, the reviewable count (eg. the red pill) is not a notification and
is located in the hamburger menu. This commit adds a new route on the server
side to retrieve the reviewable count for the current user and a ping
(refreshReviewableCount) from the client side to sync the reviewable count
whenever they open the hamburger menu.

REFACTOR: I also refactored the hamburger-menu widget code to prevent repetitive uses
of "this.".

PERF: I improved the performance of the 'notify_reviewable' job by doing only 1 query
to the database to retrieve all the pending reviewables and then tallying based on the
various rights.

app/controllers/reviewables_controller.rb

@@ -68,6 +68,10 @@ class ReviewablesController < ApplicationController
     render_json_dump(json, rest_serializer: true)
   end
 
+  def count
+    render_json_dump(count: Reviewable.pending_count(current_user))
+  end
+
   def topics
     topic_ids = Set.new