Improve bin/background_jobs script

This contains two commits:

1. Allow specifying queues for bin/background_jobs

   If you set the SIDEKIQ_QUEUES environment variable, bin/background_jobs will use that queue list rather than the default ('*'; all queues).

   This is needed for gitlab-com/gl-infra/scalability#1281 (closed).

2. Write output of bin/background_jobs to stdout and log file

   bin/background_jobs would write to stdout with start_foreground, and to log/sidekiq.log with start or restart. That was confusing, as other application log files are in log/ in development. To simplify this, use tee to make start_foreground both write to stdout and log/sidekiq.log.