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](https://docs.gitlab.com/ee/api/projects.html#list-forks-of-a-project) 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
issue