Feature to have world-accessible snippets

Created by: jackbot

I've added a new feature to snippets to allow them to be world-accessible (like the new "public projects" feature). Before, a snippet could have its private attribute as true/false - a private snippet could only be viewed by the user who created it, whereas a public snippet could be viewed by any GitLab user. I've changed the private attribute to visibility - allowing 3 visibility types: private, gitlab_public and world_public. The first two are the same as described before, but world_public lets the snippet be visible on the public page allowing anyone people to view the snippet without being signed in.