Adding disable logic for buttons.

Updated tests and introduced common toggle disable approach for registration and login. 
This is for ECOM-641
@AlasdairSwan @wedaly
I'm confused by this.  If `$formButton` is defined by the subclass, the superclass shouldn't try to access it.  Either (a) the subclasses should own the form button, including enabling/disabling logic, or (b) the subclasses should tell the superclass where to find the button, maybe by passing a selector to the constructor explicitly?