Cancel running pipelines after quota is exceeded

As raised here: https://gitlab.com/gitlab-org/gitlab-ee/issues/10690#note_156531881 we only tally and enforce pipeline quotas after they are complete / when starting a new pipeline.

This means that if someone launches 10,000 pipelines and exceeds their pipelines minutes quota immediately, all 10,000 pipelines will still run. Instead, we should cancel the pipelines, and then ~Fulfillment will inform the users that their quota is exceeded.