Default settings for repository visibility

Most people who install GitLab probably to not intend to have public access, or if they do it is not intended initially. This issue is to propose a change to the defaults to the following:

Or if not this restricted, perhaps we make the default visibility levels "private" with public limited until an admin makes a setting change.

This may avoid similar situations to https://techcrunch.com/2019/05/08/samsung-source-code-leak/