Remove project notification settings associated with deleted projects

On GitLab.com, there are 1560 project notification settings with no valid project
all created on 2016-04-15 10:38:53 for some reason. This migration should purge
those entries.

b8f38437 should prevent this issue from occurring in the first place.

Closes gitlab-com/support-forum#678

CHANGELOG

@@ -4,6 +4,7 @@ v 8.9.0 (unreleased)
   - Allow enabling wiki page events from Webhook management UI
   - Allow forking projects with restricted visibility level
   - Improve note validation to prevent errors when creating invalid note via API
+  - Remove project notification settings associated with deleted projects
   - Redesign navigation for project pages
   - Fix groups API to list only user's accessible projects
   - Redesign account and email confirmation emails

db/migrate/20160527020117_remove_notification_settings_for_deleted_projects.rb

+class RemoveNotificationSettingsForDeletedProjects < ActiveRecord::Migration
+  def up
+    execute <<-SQL
+      DELETE FROM notification_settings
+      WHERE notification_settings.source_type = 'Project'
+        AND NOT EXISTS (
+              SELECT *
+              FROM projects
+              WHERE projects.id = notification_settings.source_id
+            )
+    SQL
+  end
+end