Two-way communication with container registry via notifications API

This is a follow up issue after shipping https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10109.

Currently we do no support two-way communication with container registry. This means that the only way to know what is happening in the container registry is to subscribe to authentication events (which by a design are going through GitLab instance).

Being able to subscribe to registry events using notifications API would be helpful.

Original idea comes from @andrebsguedes who provided PoC of this in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7154.