Merge branch 'remove_is_admin' into 'master'

app/controllers/admin/application_controller.rb

app/controllers/admin/impersonations_controller.rb

app/helpers/visibility_level_helper.rb

app/models/user.rb

app/policies/ci/runner_policy.rb

app/services/users/create_service.rb

app/views/layouts/header/_default.html.haml

changelogs/unreleased/remove_is_admin.yml

lib/api/entities.rb

lib/api/groups.rb

lib/api/helpers.rb

lib/api/notes.rb

lib/api/runners.rb

lib/api/services.rb

lib/api/users.rb

lib/api/v3/groups.rb

lib/api/v3/notes.rb

lib/api/v3/runners.rb

lib/api/v3/services.rb

lib/gitlab/import_export/relation_factory.rb