Add group visibility level

Douwe Maanrequested to merge
issue_12658 into master

Supersedes https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3051

Closes #12658 (closed)

To do

  • Update Admin group edit form to set visibility level
  • See if visible_to_user and public_to_user can be combined and/or renamed
  • Don't use current_application_settings in migration
  • Make sure are tested adequately:
    • JoinedGroupsFinder
    • GroupProjectsFinder
    • GroupsFinder
    • PersonalProjectsFinder
    • Groups::UpdateService
    • Groups::CreateService
    • ProjectsFinder
    • SnippetsFinder
    • Project#visibility_level_allowed?

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