Prevent changing feature flags during incidents

This changes the "feature set" command so it checks for any ongoing incidents in the issue tracker https://gitlab.com/gitlab-com/gl-infra/production/-/issues. If there are one or more S1, S2, or S3 issues, the state of a feature flag can't be changed.

In the rare case that somebody needs to work around this, they can specify the --ignore-incidents option. When this option is set we add some additional data to the feature flag log, making it clear the user chose to ignore the incidents.

This fixes #81 (closed)