Initialize Sidekiq with the list of queues used by GitLab

The Sidekiq client API adds an entry to the Sidekiq "queues" list,
but mail_room and gitlab-shell use redis-rb directly to insert jobs
into Redis and thus do not make an extra "sadd" call to Redis
each time a job is inserted. To make it possible to monitor
these queues via the API, add an initialization step to
set up the list at startup.

Closes gitlab-com/infrastructure#682