Redirect to 2FA selection screen

Apps like 'rainloop' use \OCP\Util::isLoggedIn() to check whether the
current request is authenticated. Since we redirected to the index
page before, it resulted in an infinite redirection loop. This change
sets the redirection URL to the 2FA selection page, which is the only
allowed page in that authentication state.

Fixes https://github.com/nextcloud/server/issues/3702



Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>