Create merge requests across a fork network

What does this MR do?

This MR makes it possible to create MRs across a fork network.

It does this by creating a ForkNetwork when the first fork of a project is created. It sets the root_project of the fork network to the project it was forked from, and adds the root as a member of the network.

Then it adds the newly created fork of the project as a member of the network.

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
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #20097 (closed)