Adopt Unleash for rolling out Pajamas

Problem

With Pajamas conversion we will end up with a number of feature flags

Enabling feature flags currently is to SSH into the box and toggle the flag from the command line. This requires access to the staging and production server to toggle the flag and to query which flags are enabled.

Disabling a feature flag if something is wrong can also be time consuming.

Proposal

Unleash has solves a number of these problems.

Unleash is enabled in the Customer Portal so adopting Unleash requires updating existing feature flags (2 as of opening this issue) to the Unleash prefix: https://gitlab.com/gitlab-org/customers-gitlab-com#feature-flags-unleash

Result

Feature flags are visible in a central place: https://gitlab.com/gitlab-org/customers-gitlab-com/-/feature_flags

Next steps (if any)

Document how to use Unleash in local development