Fix: problem with notifications not dismissing due to HTTP caching

The problem was related to tying the invalidation to the association of unread notifications. Its
cache key can remain constant as you read old notifications.

https://app.box-car.com/5986089/cards/2620/edit

app/controllers/notifications/trays_controller.rb

@@ -2,6 +2,8 @@ class Notifications::TraysController < ApplicationController
   def show
     @notifications = Current.user.notifications.unread.ordered.limit(100)
 
-    fresh_when @notifications
+    # Invalidate on the whole set instead of the unread set since the max updated at in the unread set
+    # can stay the same when reading old notifications.
+    fresh_when Current.user.notifications
   end
 end