UI: Notifications for Partner Engineers and Partners

[chouseknecht]

• When a new Collection Version artifact is uploaded to AH the Partner Engineers should be notified via Slack
• After a Collection Version is changed to certified or rejected:
  • the partner should be notified via Slack and/or Email. This should be configurable for each partner
  • Any subscribers that are following a Collection should be notified via Email. Subscribers should be able to configure notifications. See issue #71

NOTE: c.rh.com offers a notification service that supports Email and/or Slack. Notification will be handled through this service.