Add webhook support to Remote Mirroring

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Right now Remote Mirroring https://docs.gitlab.com/ee/workflow/repository_mirroring.html has a default setting of polling 1 time per hour.

I think we should allow people to set a web hook in addition to that so their repo's are updated immediately.

Links:

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/web_hooks/web_hooks.md
• https://developer.github.com/webhooks/
• http://stackoverflow.com/questions/21962872/how-to-create-a-gitlab-webhook-to-update-a-mirror-repo-on-github
• https://news.ycombinator.com/item?id=12492895

/cc @JobV