Allow custom redirection after login

Passwordless automatically redirects to 'user' after a successful login, which means it won't work with any modules providing custom landing pages, or redirection after login (such as Login Redirect).

The upcoming commit introduces a hook, hook_passwordless_login_redirect_alter(), which will allow any module to customize the landing page. The assumption is that only one enabled module will implement this hook. If more than one module implements it, the heaviest one will win.