[Meta] Support really large projects, with advanced git flows, like the linux kernel git workflow

Issue idea came from: http://blog.ffwll.ch/2017/08/github-why-cant-host-the-kernel.html

Usecase

Please read this article

Subissues

• https://gitlab.com/gitlab-org/gitlab-ce/issues/36277 - Establishing the fork relationship after the fact
• https://gitlab.com/gitlab-org/gitlab-ce/issues/36275 - Multiple forks of the same repo in one organization
• https://gitlab.com/gitlab-org/gitlab-ce/issues/36280 - Be able to move merge requests across multiple projects/forks
• https://gitlab.com/gitlab-org/gitlab-ce/issues/36283 - Be able to create a copy of merge request when editing and selecting different target branch
• https://gitlab.com/gitlab-org/gitlab-ce/issues/36279 - Unify/Connect issue/mr discussions across copies

Value

Being able to support really large projects, with advanced git flows, like the linux kernel git workflow