Allow to fork repository into the same owner (#32819)

This feature is experimental, not fully tested, and may be changed in
the future.

It is only designed for users who really need it: set
`[repository].ALLOW_FORK_INTO_SAME_OWNER=true` in your app.ini

Doc: https://gitea.com/gitea/docs/pulls/122


![image](https://github.com/user-attachments/assets/38d08c23-9cfc-49d8-9321-ff81edf65395)

modules/setting/repository.go

@@ -53,6 +53,7 @@ var (
 		AllowDeleteOfUnadoptedRepositories      bool
 		DisableDownloadSourceArchives           bool
 		AllowForkWithoutMaximumLimit            bool
+		AllowForkIntoSameOwner                  bool
 
 		// Repository editor settings
 		Editor struct {