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

test/integration/registrant_area/sign_in/id_card_test.rb

@@ -0,0 +1,31 @@
+require 'test_helper'
+
+class RegistrantAreaIdCardSignInTest < ApplicationIntegrationTest
+  setup do
+    allow_business_registry_component_reach_server
+  end
+
+  def test_succeeds
+    post_via_redirect registrant_id_card_sign_in_path, nil,
+                      'SSL_CLIENT_S_DN_CN' => 'DOE,JOHN,1234',
+                      'SSL_CLIENT_I_DN_C' => 'US'
+
+    assert_response :ok
+    assert_equal registrant_root_path, path
+    assert_not_nil controller.current_registrant_user
+  end
+
+  def test_fails_when_certificate_is_absent
+    post_via_redirect registrant_id_card_sign_in_path, nil, 'SSL_CLIENT_S_DN_CN' => ''
+
+    assert_response :ok
+    assert_equal registrant_id_card_sign_in_path, path
+    assert_nil controller.current_registrant_user
+  end
+
+  private
+
+  def allow_business_registry_component_reach_server
+    WebMock.allow_net_connect!
+  end
+end