Add application settings changes to audit events
We have instance-level audit events: https://docs.gitlab.com/ee/administration/audit_events.html#instance-events
These currently don't include changes to application settings. On GitLab.com and our staging environment, there is a sizeable group of people who can change these settings. Mostly they don't do that, but if they did, it could have a considerable impact.
In gitlab-com/gl-infra/production#3010 (comment 445838346) for instance, we noticed a setting was different in staging compared to production. We don't know:
- Why it was different.
- When it was changed.
- Who changed it.
An audit log can't answer the first question, but it can answer the second two, which would already be a huge help. It would also let us auto-create review issues every time we change a setting, to validate that we had the appropriate change issue in place to do so.