Merge branch 'feature/sidekiq-cron-config' into 'master'

Sidekiq-cron configuration moved to gitlab.yml

When `sidekiq-cron` was introduced, jobs configuration where placed
in `schedule.yml` file.

As discussed in #3928, this is not desirable. 
We moved it to `gitlab.yml`, exposing only the "cron" part of the configuration.

See merge request !2087