forking a repository within the same group only possible through the API
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### Proposal
When we try to fork a repository in a namespace, the UI doesn't allow us to create the fork in the same namespace, but the API does.
Steps to reproduce:
- create a namespace
- create a repository
- click on fork on this repository
- the `project URL` shows all the namespace the user has access, but doesn't show the namespace where the repository is
- if we use an API call `curl --request POST --data "namespace_id=NAMESPACE_ID" --data "name=REPO_FORK" --data "path=REPO_FORK" --header "PRIVATE-TOKEN: <YOUR_TOKEN_HERE>" "https://gitlab.com/api/v4/projects/PROJECT_ID/fork"` this works, we can fork it in the same namespace with a different name
A silver customer has been requesting this: [internal link](https://gitlab.zendesk.com/agent/tickets/229233)
issue