fixing signup/signin views

The user registration page is not directly managed by devise (note the :registerable module is not included in the user model devise call) since we are making the registration process easier by not requiring a password (this is randomly generated and mailed to the user, if you remember).

That's the reason why the _links partial explodes with those errors. To fix this I added 3 helpers methods in the users_controller to define the devise variables `devise_mapping`, `resource_name` and `resource_class`.

Another problem was the link to signup not being displayed in the signin view. To fix this I changed the logic by checking if the current controller is not "users_controller" (which is used for register a new user) in which case I want to render the signup link.

I added 2 specs to check the rendering of the signup/signin links in the signin/signup views and made some changes to the existing specs to reflect the changes in the UI.