Add two factor auth to core

apps/dav/lib/Connector/Sabre/Auth.php

core/Application.php

core/Controller/LoginController.php

core/Controller/TwoFactorChallengeController.php

core/Middleware/TwoFactorMiddleware.php

core/css/styles.css

core/routes.php

core/templates/twofactorselectchallenge.php

core/templates/twofactorshowchallenge.php

lib/private/App/AppManager.php

lib/private/App/InfoParser.php

lib/private/AppFramework/DependencyInjection/DIContainer.php

...vate/Authentication/Exceptions/LoginRequiredException.php

...hentication/Exceptions/TwoFactorAuthRequiredException.php

...uthentication/Exceptions/UserAlreadyLoggedInException.php

lib/private/Authentication/TwoFactorAuth/Manager.php

lib/private/Server.php

lib/private/User/Session.php

lib/public/Authentication/TwoFactorAuth/IProvider.php

tests/Core/Controller/LoginControllerTest.php