Allow disabling Spree Frontend

Allow disabling Spree Frontend

(NOTE: We're debating the best long-term solution for this over [here](https://github.com/spree/spree/pull/5360) but I didn't want that 

Via a config preference.

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 

We can also try to make sure that we prevent requests at the nginx layer, but this seems like a nice way to make sure we really close of

Conflicts:
    frontend/app/controllers/spree/store_controller.rb