Allow Devise modules to register multiple routes

[abevoelker]

This change was precipitated by https://github.com/abevoelker/devise-passwordless, a Devise extension which adds a :magic_link_authenticatable strategy which enables registration and sign-in via emailed magic links.

This change is needed we can register both a :magic_link and a :session route and controllers, because we need to both provide a custom sessions controller as well as handle magic links. We are currently monkey patching this logic in and it works great - Devise.add_module continues to work exactly the same for modules with singular routes.