Add webhook support to Remote Mirroring
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=1011) </details> <!--IssueSummary end--> 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
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