feat: allow forking added to the repository settings

[pmartindev]

Resolves #353

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Before the change?

• Current behavior does not allow changing the repository setting to allow/disallow forking.

After the change?

• Repositories can now (optionally) be set to allow forking. This only applies to private and internally owned repositories. This setting cannot be changed on publicly owned repositories as public repositories can always be forked.

Pull request checklist

• Schema migrations have been created if needed (example)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[pascal-hofmann]

See #1649 :(

Edit: This also resolves #1071.