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 :)
We can have the case where there is no login ticket, cannot we? If so, I would rename the method as something like ```checkLoginTicketIfExists``` (feel free to find a better name).