current_store on spree frontend (+ fixes some frontend specs)

Another baby step towards multitenancy.

This fixes [two failing frontend specs](http://ci.spree.fm/viewLog.html?buildId=50240&buildTypeId=bt78&tab=buildLog#_focus=1655) due to storing the "default site" across test cases. Also greatly reduces deprecation warnings in frontend specs.

This adds `current_store` to `Spree::StoreController` and exposes it as a helper method.