Merge branch 'sh-disable-sidekiq-session' into 'master'

Disable the Sidekiq Admin Rack session

See merge request gitlab-org/gitlab-ce!21441

changelogs/unreleased/sh-disable-sidekiq-session.yml

+---
+title: Disable the Sidekiq Admin Rack session
+merge_request: 21441
+author:
+type: security

config/initializers/sidekiq.rb

+require 'sidekiq/web'
+
+# Disable the Sidekiq Rack session since GitLab already has its own session store.
+# CSRF protection still works (https://github.com/mperham/sidekiq/commit/315504e766c4fd88a29b7772169060afc4c40329).
+Sidekiq::Web.set :sessions, false
+
 # Custom Queues configuration
 queues_config_hash = Gitlab::Redis::Queues.params
 queues_config_hash[:namespace] = Gitlab::Redis::Queues::SIDEKIQ_NAMESPACE