Web hook fork event

Problem to solve

When a team uses a forking workflow, I want to perform automation whenever a new fork is created, but it's impossible for me know when a fork has been created unless I poll the project forks API or use a system hook for project creation. I want a simple web hook I can subscribe to.

Further details

Proposal

Add a web hook that emits an event whenever a new fork is created

Permissions and Security

The permissions implications would be simplified by https://gitlab.com/gitlab-org/gitlab-ce/issues/8935 since the owners/maintainers of the upstream project would inherit access to private forks, and thereby the fork even wouldn't be leaking any information.

What does success look like, and how can we measure that?

Number of projects with fork even web hooks.

Links / references

https://developer.github.com/v3/activity/events/types/#forkevent