Exclude most jobs when triggered by feature flag definition changes

This excludes all but the *:instance jobs when a pipeline is triggered by a GitLab MR that changes feature flag definition files.

This means we'll have a reasonable amount of coverage (via about 5 jobs) without overloading the runner (with about 80).

I needed to refactor the CI code so that rules could be merged. I based the style on the GitLab CI code.

See https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1061

Note: The package-and-qa jobs started by feature flag changes are currently manual and should stay so until we increase the number of jobs we can run.