Refactor ID card sign-in

- Extract to Devise custom strategy
- Use `SSL_CLIENT_S_DN_CN` env variable instead of `SSL_CLIENT_S_DN` to
get ID card data
- Remove `database_authenticatable` strategy from `RegistrantUser`

Closes #1047

lib/devise/models/id_card_authenticatable.rb

@@ -0,0 +1,7 @@
+module Devise
+  module Models
+    # Devise fails without this module (and model: false does not help)
+    module IdCardAuthenticatable
+    end
+  end
+end