Don't select backend only shipping rates by default

As frontend users shouldn't select shipping rates only meant to be used
via backend. So Estimator by default only marks frontend/generic
shipping rates as selected

Fixes #3287

While working on this fix I wondered if we really need the `display_on` field at all. Maybe a boolean backend flag on shipping methods should be enough in case there's no reason to hide any shipping method in backend. Makes sense? I could put that refactoring in the my not priority list and work on it some time later.

CI is green.