Allow sidekiq to react to becoming a Geo primary or secondary without a restart, reactive

What does this MR do?

Sidekiq runs a set of cron jobs. This set differs according to whether the node is a Geo primary or secondary, or if Geo is disabled. Previously, the state was determined once, when sidekiq was started. Now, the state is checked whenever a cronjob is enqueued. If the cron is appropriate for the node type, it is added.

Are there points in the code the reviewer needs to double check?

The post-deployment migration is a bit of a cheat, but we need to re-enable these jobs as a one-time operation (so they can be manually disabled later if the administrator so desires).

Why was this MR needed?

In HA scenarios, ensuring every node is restarted in a timely manner is not easy. Much better to respond to changes without a restart being required.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Related to #4048 (closed)