Refactor vendor model

This PR cleans up a bit in the vendor model, and streamlining a few processes along the way.

Namely, the Yelp details are now in a form that is compatible directly with our vendor model. This makes it simple to update the details. Also, the confirm_details is now form, so vendor can alter it incase the information is incorrect. Originally, I intended on using the customers#update method to handle the controller logic of update, but it was not playing well with Devise. I think at this point the update_with_yelp controller method is the most parsimonious, but we may want to consider integrating it with devise in the future.