WIP: Make it possible to send merge request across different forks

Closed Bob Van Landuyt :neckbeard: requested to merge bvl-fork-network into master

What does this MR do?

This MR makes it possible to send merge requests across forks multiple levels deep.

Does this MR meet the acceptance criteria?

  • Changelog entry added, if necessary
  • Documentation created/updated
  • API support added
  • Tests added for this feature/bug
  • Review
    • Has been reviewed by UX
    • Has been reviewed by Frontend
    • Has been reviewed by Backend
    • Has been reviewed by Database

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20097

Edited by Bob Van Landuyt :neckbeard: