Break up AuthenticationViaFormAction for extensions

Deals with https://github.com/Jasig/cas/issues/723

Nothing of interest really is put into this model. Just makes it easier for subclasses of `AuthenticationViaFormAction` to have access to individual bits in the class. There's no point in a non-final class if pretty much everything interesting in it is final or hidden :)
I would have moved this method at the top of the class as it presents the global logic. And the other methods above just to help reading.