Restrict access when restricted visibility level is public

Some organizations prefer to have all Gitlab related data private
but it is currently possible for non-authenticated users to access
certain pages e.g /public and /explore

This change makes it possible to restrict access by
checking if `public` is included in the restricted visibility
level setting.

* require authentication for /explore, /help, and /public for non-authenticated users.
* remove the Explore link from the sign in page.
* redirect an unauthenticated user attempting to access /help to https://docs.gitlab.com/.