Improve repository fork dialog

[xoxys]

• Gitea version (or commit ref): v1.14.4
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No

Description

If a user forks a repository to its user space for the first time, the fork dialog is pretty clear, the new owner will be the forking user. But if a user have already an active for of a repository, the fork dialog might be confusing as now the owner in the dialog is one of the organization the user might be a member of. That's confusing for some users, and they are wondering why they can't fork to its user space anymore. It might be even more confusing if the user has renamed the forked repo and forget about it. If the user now checks his own repos, he can't find a repo with the name.

On GitHub, this dialog (and especially this situation) is more clear as you will get notified that you have already an active fork, and you will also see the name, which is useful as it will show the repo name of the fork even if it was renamed.

I think it would be useful to adapt it in some way for Gitea to improve the ux.