Adapt helpers to use policy instead of `admin?`

app/helpers/application_helper.rb

app/helpers/import_helper.rb

app/helpers/projects_helper.rb

app/helpers/search_helper.rb

app/helpers/users/callouts_helper.rb

app/helpers/users_helper.rb

app/helpers/visibility_level_helper.rb

ee/app/helpers/ee/dashboard_helper.rb

ee/app/helpers/ee/import_helper.rb

ee/app/helpers/ee/subscribable_banner_helper.rb

ee/app/helpers/ee/users/callouts_helper.rb

ee/app/helpers/license_monitoring_helper.rb

ee/app/helpers/push_rules_helper.rb

...tures/admin/admin_show_new_user_signups_cap_alert_spec.rb

ee/spec/helpers/ee/dashboard_helper_spec.rb

ee/spec/helpers/ee/import_helper_spec.rb

ee/spec/helpers/ee/subscribable_banner_helper_spec.rb

ee/spec/helpers/ee/users/callouts_helper_spec.rb

ee/spec/helpers/license_monitoring_helper_spec.rb

ee/spec/helpers/projects_helper_spec.rb