Configure sidekiq-cron via gitlab.yml

In !1982 (merged) we added a hard-coded config/schedule.yml file. I think it would be better if the schedule was defined in config/gitlab.yml.

• GitLab already has way too many config files
• sidekiq-cron does not force us to use that file, we wrote our own initalizer that sucks in config/schedule.yml
• if we do add a config file it should be a .example file so that people can edit it without losing their changes during upgrades from source

The simplest thing is just to put this configuration in gitlab.yml somewhere.

cc @brodock @dzaporozhets @vsizov