Separates sign_up and sign_in on ConfirmationsController

Currently the RegistrationsController allows you to hook into the sign_up flow by overriding sign_up in that controller. This work was done in 

https://github.com/plataformatec/devise/pull/2111

If you were to use the confirmable module, this hook does not work because the sign_up call gets bypassed when resource.active_for_authentication? gets called (and returns false). 

We would be able to reintroduce this hook by using the same pattern in the ConfirmationsController and allowing it to be overridden there instead. It also makes things more consistent and is a minor code change.

Hope this works! 

p/s Thanks for the work on Devise, it is awesome!