Fix validations related to mirroring settings form.

Some of our users are changing to GitLab as their main repository, it means that they're disabling the local mirror and setting up a remote mirror instead. This fix helps to avoid the extra steps required in order to achieve the desired configuration.

Closes: gitlab-com/support-forum#1044 (closed) gitlab-org/gitlab-ee#1062

Related: gitlab-org/gitlab-ce#17940

Also the UI has been modify to have a single update button:

Before

After