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

app/views/registrant/sessions/new.html.erb

@@ -11,7 +11,7 @@
         <%= link_to '/registrant/login/mid' do %>
             <%= image_tag 'mid.gif' %>
         <% end %>
-        <%= link_to '/registrant/id', method: :post do %>
+        <%= link_to registrant_id_card_sign_in_path, method: :post do %>
             <%= image_tag 'id_card.gif' %>
         <% end %>
     </div>

app/views/registrar/sessions/new.html.erb

@@ -23,7 +23,7 @@
             <%= image_tag 'mid.gif' %>
         <% end %>
 
-        <%= link_to '/registrar/id', method: :post do %>
+        <%= link_to registrar_id_card_sign_in_path, method: :post do %>
             <%= image_tag 'id_card.gif' %>
         <% end %>
     </div>