WIP: Resolve "[FE] Restrict private project forks, either completely or to private forks only"

What does this MR do?

Adds a new section Forks for private projects under Repository in Project Settings -> General -> Permissions:

• Dropdown options:
  • 'Allow public forks'
  • 'Only allow private forks'

Switching the toggle off will disallow all forks. Switching the toggle on will give users the option between allowing all forks, or only private ones.

How to test

Enable the following feature flags in the Rails console (rails c):

• Feature.enable(:show_forking_access_level)

Then select a private project -> Project Settings -> General -> Visibility, project features, permissions:

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10663

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/59030

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #59030 (closed)