Port `read_cross_project` ability from EE

Bob Van Landuytrequested to merge
bvl-external-auth-port into master

This ports the changes from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3709 to CE:

Main changes ported:

  • Helper methods for rendering menus
  • single instance finder methods on Finder classes
  • Cross project restrictions, although they aren't used in CE.

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