Improve FF change logging and link to rollout issue in logging issue

This MR improves the logging of feature flag changes and make it consistent between https://nonprod-log.gitlab.net/app/discover and https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?sort=created_date&state=closed.

This also adds a link to the rollout issue in the logging issue.

Since the MR fetches the feature flag definition, this will enable future work, e.g. Get feature flags enabled by default (#116 - closed) and Cross-link actual feature flag state change to ... (#168 - closed) / The feature flag change could push notification... (gitlab-org/gitlab#290770 - closed).

Closes Consider requiring an issue link when making ch... (#97 - closed), Fix feature flag log issue scopes (#145 - closed).