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 gitlab-org/gitlab-ee#1062

Related: gitlab-org/gitlab-ce#17940

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

#### Before
![1](/uploads/83c1d614a35911740154d4167f77f6a0/1.png)

#### After

![2](/uploads/87f73a32c35d7fe83558932cdc1a21d7/2.png)

See merge request !773