move account notification queries to slave

closes #LA-879

Change-Id: I59029d363796b6ddabd25a01a43d4abe86548497
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/232677
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Clint Furse <cfurse@instructure.com>
QA-Review: Clint Furse <cfurse@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

app/models/account_notification.rb

@@ -67,6 +67,7 @@ class AccountNotification < ActiveRecord::Base
   end
 
   def self.for_user_and_account(user, root_account)
+    Shackles.activate(:slave) do
       if root_account.site_admin?
         current = self.for_account(root_account)
       else
@@ -127,8 +128,10 @@ class AccountNotification < ActiveRecord::Base
         # applicable, they probably need to be cleared out.
         current_ids = current.map(&:id)
         if !(closed_ids - current_ids).empty?
+          Shackles.activate(:master) do
             user.set_preference(:closed_notifications, closed_ids & current_ids)
           end
+        end
         current.reject! { |announcement| closed_ids.include?(announcement.id) }
 
         # filter out announcements that have a periodic cycle of display,
@@ -149,6 +152,7 @@ class AccountNotification < ActiveRecord::Base
 
       current
     end
+  end
 
   def self.for_account(root_account, all_visible_account_ids=nil)
     # Refreshes every 10 minutes at the longest