Bidirectional mirroring blocked by URL validation

Using the same URL for push and pull mirroring isn't possible because of validation errors

https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/remote_mirror.rb#L181

Steps to reproduce

• Create a new project, push a repo to it that exists on a different git server
• Enable pull mirroring and add the upstream git repo URL
• Attempt to enable push mirroring, not possible