Allow editing the URL a project is mirrored from
When a project is mirrored from a remote location to a local pagure
instance, we so far had no way to edit this url, for example for
when the upstream project changes location.
With this commit we're able to fix this.
Fixes https://pagure.io/pagure/issue/4647
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>