Rake task to enable work item epics

What does this MR do and why?

Issue: #499529 (closed)

This adds the following two rake tasks:

• gitlab:work_items:epics:enable
• gitlab:work_items:epics:disable

To either enable or disable the work item epics experience. When enabling work item epics for the instance, we validate if the data is in sync. If the data is not in sync, we do not allow to enable the new experience.

Instances that cannot enable the new experience, need to wait for work item epics to be GA.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Case	Screenshot
Successfully enabling work item epics	verified.mov
When an epic is out of sync	verification_failed.mov
Disabling

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Run the following rake tasks to enable and verify or disable epic work items.

bundle exec rake gitlab:work_items:epics:enable

bundle exec rake gitlab:work_items:epics:disable