* Totally refactor includes/specials/SpecialUserlogin.php:

* Totally refactor includes/specials/SpecialUserlogin.php:
** Split backend stuff out into includes/Login.php
** Split account creation and login stuff up, into includes/specials/CreateAccount.php and includes/specials/Userlogin.php.
* Reimplement the special pages as subclasses of SpecialPage
* Use HTMLForm to generate the input forms
** Deprecate and delete includes/templates/Userlogin.php, which is horrible and old :D

This changes the syntax of the UserLoginForm and UserCreateForm hooks, and AuthPlugin::modifyUITemplate; they now receive the SpecialPage subclass rather than the template to work with. Update everything I could find in SVN to accommodate this.