Normalise sidekiq setup

This brings the sidekiq setup here in line with how we configure it in other apps.

* Use a `redis.yml` to set redis options
* Use a `sidekiq.yml` to set sidekiq options
* Launch sidekiq from a `Procfile`

**Note:** https://github.gds/gds/alphagov-deployment/pull/749 should be merged at the same time as this.