Serve integrations pages in org admin area

Part of &22859.

Problem

Integration defaults have no pages under /o/:organization_path/admin.

On GitLab.com the instance pages are hidden by the !Gitlab.com? check in integrations_helper.rb.

Change

• Add integrations routes to organization_admin.rb.
• Make Admin::IntegrationsController and the integrations action in admin/application_settings_controller.rb use Current.organization under /o/.
• Replace the !Gitlab.com? gate with the new permission.
• Add a Settings menu item in settings_menu.rb.

Done when

• An organization owner can open and save integration defaults under /o/:organization_path/admin.

Status

Depends on: #631287 (closed), #631290 (closed)

Can start now: no