Allow disabling Spree Frontend

Via a config preference.

Let me know if there's already a better way to do this.

We use our own frontend along with spree-api and this change will allow us to easily disable the frontend on our prod boxes yet keep it enabled for development so that we can test that our spree changes are meshing well with the spree frontend code.

We could also try to make sure that we prevent requests at a lower (e.g. nginx) level, but this seems like a nice way to make sure we really close off access to frontend so that we don't end up with customers accidentally hitting frontend and getting really confused.  Setting up a config will also allow other extensions (like spree_auth_devise) to read the same setting.