Send Slack message containing diff of deployment

What does this MR do?

1. Call CoordinatedPipelineDiffsNotification from CoordinatedPipelineNotification.

   • This will start sending a Slack message containing the diff of all components being deployed.

   • This message will be sent as a threaded reply to all <env> started deployment notifications.

   • This change is behind a feature flag: diff_notifications. I've not created the feature flag yet.

2. Raise error in CoordinatedPipelineDiffsNotification if latest deployment is newer than source.

   The latest deployment should never be newer than the version being currently deployed. If it does happen for some reason, raise an error.

gitlab-com/gl-infra/delivery#2165 (closed)

Author Check-list

• [-] Has documentation been updated?