Audit Events: Log feature flag enable/disable
### Problem to solve
Feature flag changes can affect the security of users and organizations. However, they are not logged as audit events. This was [reported by a Premium customer](https://gitlab.com/groups/gitlab-org/-/epics/476#note_558202317) and also [noted by GitLab internally](https://gitlab.com/gitlab-com/chatops/-/issues/94).
### Proposal
Feature flag enable and disable should be logged as individual [Audit Events](https://docs.gitlab.com/ee/administration/audit_events.html).
epic